This class is a little overpowered; it's supposed to be.  Notes for altering it to a balanced version can be found at the bottom.

A PDF can be downloaded here: Occultist - An Alternate Warlock.pdf

Sources include the following (in addition to the ACKS rules referenced here):

http://autarch.co/comment/14305#comment-14305

http://autarch.co/forums/house-rules/spell-points-acks

Player's Option: Spells & Magic (AD&D 2E)

Tome of Magic (AD&D 2E)

I know ACKS already has a warlock class, but that's what I call this internally, and I didn't feel like finding and replacing everything from 'warlock' to 'occultist'.  I named the page Occultist to denote that this is not intended to replace the existing warlock class, it's a different spellcasting method entirely.

Warlock
Prime Requisite: Int
Requirements: Int 9
Hit Dice: d4
Maximum Level: 14

Because of their devotion to the study of forbidden lore, warlocks receive limited combat training. At first level, warlocks hit an unarmored foe (AC 0) with an attack throw of 10+. They advance in attack throws and saving throws only two points every six levels of experience (i.e., half as fast as as fighters). They are unable to use shields, fight with two weapons, or wear any kind of armor.  Warlocks may only fight using a dagger, dart, club, or sling.

In exchange for these weaknesses, warlocks can learn and cast powerful spells. A warlock is not directly limited in the number of spells he can cast per day, but he is limited in his number of spells known. A warlock may cast only those spells in his repertoire. He may have a number of spells in his repertoire equal to the number of spells per day listed on the Mage Spell Progression Table (ACKS Core page 20), plus his Intelligence modifier.  Warlocks use a different spell list than mages; see here, Warlock Spells.

Warlock spells are innately corrupting. Any warlock character has a Corruption score. A warlock's Corruption score starts at zero at character creation. Every time you kill someone from damage with a warlock spell, you gain one Corruption. The first time in each month you cast a warlock spell, you gain one Corruption. Whenever you instantly kill someone with a warlock spell (such as Finger of Death or Death Spell), you gain two Corruption.

As long as your Corruption is below half your Wisdom score, you are untainted. You receive no bonuses or penalties. You may reduce your Corruption score by 1 by spending a week in meditation and spending 500 GP on incense and ritual cleansing items.

When your Corruption is greater than half your Wisdom score, you become tainted. A tainted character receives a +1 bonus on his magical research throws to cast warlock spells. However, your Strength and Constitution scores are reduced by two points each. A tainted character may not reduce his Corruption score without the assistance of an atonement, geas, or similar spell cast by a lawful cleric. In addition, when a character becomes tainted, he must roll 1d10 on the Magical Mutations sub-table (Player's Companion 109).

When your Corruption is greater than your Wisdom score, you are dark. A dark character receives a +2 bonus on his magical research throws to cast warlock spells. However, you lose an additional 2 points of Str and Con (for a total of -4 Str/Con). Every time a dark character levels up, you must save vs death; if you fail, you lose an additional point of both Str and Con. In addition, when a character becomes dark, he must roll 2d10 on the Magical Mutation sub-table.

A warlock's powerful arcane spells are not without risk. In order to cast a spell, the warlock must make a magical research throw. Twice the level of the spell is applied as a penalty; the warlock's Int bonus is applied as a bonus. A further bonus or penalty is applied based on the amount of time spent casting the spell:

Using precious materials adds a bonus of +1 per 100 gp of precious materials, to a maximum of 200 gp per level of the spell. Use of precious materials cannot be done on a spellcast taking less than one round.

Using special components (such as the heart of a hellhound for a fireball spell) adds a bonus of +1 per 100 GP value of special components (again, to a maximum of 200 GP per level of the spell). Use of special components cannot be done on a spellcast taking less than one round.

The sacrifice of a living creature can be used to grant the warlock a bonus of +1 per HD of the sacrificed creature. Sacrifice cannot be used as a component for a spellcast taking less than one turn. (Note that multiple creatures may be sacrificed. The maximum bonus from sacrifice is equal to four times the level of the spell.)

A warlock may use a focus (described later) to grant himself a bonus when spellcasting.

Regardless of modifiers or target number, an unmodified result of 1-3 on the d20 always fails.

Should a warlock succeed in his throw, his spell is successfully cast. On a failure, the cost for the spell is drawn from his spell points. If he cannot pay the cost in spell points, he loses all of his spell points, then loses 2 HP for each spell point he cannot pay. In addition, one random weakness per spell level triggers, and also, a wild surge occurs.  For specific results of a wild surge, see here: Wild Surges.

A warlock's spells are powered by the life energy of living things, usually unwillingly. A warlock character may store up to a maximum number of spell points given on the Warlock table. He is capable of tapping his own life energy without harming himself, and so these spell points will slowly regenerate; a full night's rest will grant the warlock 1 spell point for each level of spell he is capable of casting. (That is 1 spell point at 1st or 2nd level, 2 at 3rd or 4th, 3 at 5th, and so on). As a cursory glance at the table and the cost of spells will tell you, this is not nearly enough to power his spells. A warlock may also gain spell points through sacrifice, blood, areas of natural power (such as ley lines), death (even if not sacrifice), and at the highest levels, may draw them directly out of the bodies of his foes as an attack form.

At first level, a warlock may only gain points through sacrifice and blood. He may sacrifice living creatures in a ceremony requiring no less than 1 turn per creature; the creature must be helpless or willing throughout the entire procedure. He must then make a magical research throw; if he succeeds, he gains spell points equal to twice the hit dice of the sacrificed creature. (0-level humans are considered 1/2 HD.)

He may also bleed living creatures (including himself) for power, requiring only one round. The creature must be helpless or willing for the entire round. The warlock must declare how many spell points in blood he wishes to extract from the creature, then must make a magical research throw. Whether the magical research throw succeeds or fails, the creature takes 1 damage per spell point. If the magical research throw succeeds, the warlock gains the amount of spell points declared, to a maximum of the creature's HD. This damage cannot be healed by any magic weaker than restore life and limb past 1 HP (a character who is bled for power may be healed up to 1 following all normal rules, but all subsequent hit points must be restored non-magically or via extremely powerful magic.) (Note that the damage is not capped; if the warlock wishes to try to extract six spell points from a goblin, the goblin will almost certainly die, and the warlock will gain at most a single spell point.) (Side note: This sounds OP when you think that the warlock can just declare a million and do a million damage to the target. But a helpless target can be automatically slain in one round anyway, so anyone can do that.)

A warlock may craft a focus to aid in his magical research throws to cast spells. At first level, he may only craft lesser foci. A lesser focus is a one-use item that costs 100 gp per focus level and takes 1 day per level to create; it may have a maximum focus level equal to the maximum level spell the warlock can cast. (Thus, at first level, that's a max focus level of 1; at 11th level, it's a max of 6). A lesser focus gives a +1 bonus per level of the focus when casting a specific spell defined in the focus. (An example of a lesser focus would be smoke powder, level 2 focus: grants a +2 bonus when casting wall of smoke.) Creating a lesser focus requires a magical research throw using all the rules for creating a scroll. (Note that 100 gp/level is the base cost; if you have a sample, it costs only 50 gp/level.)

At 3rd level and every two levels thereafter (when the warlock learns a new level of spells), he must roll 2d20 + level on the Weakness table, to reflect the corrupting influence of his forbidden lore. (Even those warlocks who have managed to remain untainted, somehow, must roll.)

For specific results of rolls on the Weakness table, see here: Weaknesses.

Beginning at 5th level, a warlock may draw spell points out of areas of natural power. The exact power of any area of natural power is up to the Judge, but it will always be expressed in terms of spell points per day. A table with recommendations of scale follows. (Note: Sinkholes of evil rate as 3 spell points/day, blighted sinkholes as 6/day, and forsaken sinkholes as 15/day or 21/day at the Judge's discretion.)
1 spell point/day: Ordinary ley line. Likely found all over the place in the campaign world.
3 spell points/day: Large ley line. Likely found occasionally, much like large rivers.
6 spell points/day: Ordinary node (intersection of 2 or 3 ley lines). Likely found rarely; three to five per continent.
10 spell points/day: Large node (intersection of 4 or more ley lines). Likely found extremely rarely, one to two per continent.
15 spell points/day: Exceptional confluence, usually the influence of another plane or created by exceptionally powerful magic or events. Likely found vanishingly rarely, one or two in the world.
21 spell points/day: World-shattering confluence; natural power of this magnitude is not a consistent thing and is usually the result of an extremely powerful extraplanar being intentionally feeding power into the world. One might exist in the world temporarily for a few years every century, or even less often.

A warlock who draws energy from an area of natural power may store more spell points than his normal cap, so long as he remains within the area of natural power, up to a maximum of double his normal cap. If he leaves the area, he will begin to lose spell points over his normal cap, at the rate of 10% per hour.

Also at 5th level, a warlock may begin to create greater foci. Greater foci are like lesser foci, but their bonus applies to a category of spells (for example, summoning spells or necromancy spells). A greater focus costs 500 gp per level and takes 1 week per level to create.

Thirdly at 5th level, a warlock may create scrolls and potions as a mage can.

Beginning at 7th level, a warlock may gain power out of the death of living creatures near him. Each time a creature dies within 10' per level of the warlock, he gains 1 spell point per 2 HD of the creature. (Normal maximum spell points apply.)

Beginning at 9th level, a warlock may create permament foci. A permament lesser focus costs 10,000 GP per level and takes 50 days per level to construct. A permament greater focus costs 25,000 GP per level and takes 100 days per level to construct. They otherwise follow all rules for creating magic items.

Also at 9th level, a warlock may make permament magic items such as rings, staffs, wands, and so on.

Beginning at 11th level, a warlock may cast powerful ritual spells of 7th, 8th, and 9th level as a mage can (his own spellcasting rules still apply, but he learns them through research and is limited in rituals known like a mage).

Beginning at 14th level, a warlock may draw power for his spells directly out of his foes while they are still alive. Once per day, a warlock may take a full round to gather power. All living creatures within 60' of the warlock suffer 10d6 damage, and the warlock gains up to 5 spell points per creature injured, to a maximum of the creature's HD.

Notes to alter to balanced version:

This class is designed for play in a world where it is hunted to extinction by almost everyone who knows of their existence.  Even incredibly evil, chaotic, mass murderers will band together with paladins to hunt them down, and the paladin will say "ok, the mass murderer isn't that bad, we'll work together on this".  This is the kind of disadvantage that I allowed to leverage the addition of more custom powers.

If you want to use it in a world where they are much more normal spellcasters (even just evil spellcasters):

-Corruption gives no bonuses, only the penalties/mutations.

-Empowerment (Death) and Empowerment (Essence Theft) are removed entirely.

-Spell points are no longer gained past level 11; 90 is the most any member of this class has access to.

-Empowerment (Natural) does not allow you to exceed your spell point cap.

Putting this here so there is a link: Flame Knight Spell List