My favorite version of the Warlock class is the Hexblade from the D&D 4E Essentials line. I love the flavor of an arcane sword-wielding class with powers gained from a pact with an otherworldly, enigmatic entity. To my mind, the key features of a Hexblade are the pact, the pact weapon and the eldritch blast. The eldritch blast is an at-will power in 4E, but for old-school play I envision it more as something to be used once per day at first level, with more uses per day coming at higher levels. I also think it could be modified to do different kinds of damage or effect multiple enemies through invocations.
As far as the pact and pact weapon go, I see the Hexblade being somewhat close to a Thief or Cleric in a combat role: more lightly armored than a Fighter, but more heavily armed than a Magic-User. A second-line combatant, but with potential to do some impressive damage. Where the Thief has the sneak attack, the Hexblade has a pact weapon.
This post outlines the foundation of the class as I see it. More concrete mechanics and stats (saving throws, hit dice, base attack bonus, etc.) will be coming in the next post. For now, I just wanted to write down my initial thoughts on how I'd like this new-school class to be expressed in an old-school system. I welcome any critique or comments, so please, feel free to comment below.
At first level, Hexblades will have to choose a pact. This pact represents where a Hexblade's power comes from. Infernal pacts are typically made with fiendish beings - a classic "sell your soul" scenario, perhaps. The Fey pact, on the other hand, is generally made with faerie creatures, such as sprites, dryads, pixies, or other strange, otherworldly creatures.
Each pact grants different invocations. The Infernal pact grants access to more overtly damaging invocations, with an emphasis on flame and death. The Fey pact, on the other hand, grants invocations that allow for more mind-affecting alterations to the eldritch blast - adding a confusion effect, for example.
The pact weapon is the signature feature of the Hexblade. Based on the individual Hexblade's pact, the pact weapon is not only the Hexblade's primary weapon, but a symbol of the Hexblade's power source. Similar to a Magic-User's spellbook, the Hexblade starts play with a pact weapon matching their pact.
Hexblades have to be strong (to wield their pact blade) and have impressive force of personality (to channel arcane power for their eldritch blast and invocations). I'm going to tentatively set ability restrictions of Strength 9 and Intelligence 9, with an experience bonus of 5% if either score is 13+ or 10% if both scores are 13+.
A Hexblade's invocations are similar to spells, but instead of being stand-alone effects, they affect the eldritch blast instead. Invocations can be used as many times per day as the Hexblade has uses of their eldritch blast. Hexblades don't have to prepare invocations - any time they can use their eldritch blast, they can use any invocation they know on it.