After spending some time thoroughly reading through the character class options in the 5e PHB, something that really sticks out is the Warlock class. The Warlock is a magic user who gains access to spells and other powers by making a pact with a powerful entity, but not a god like a cleric or paladin. The player can choose from one of three entities as a patron; an archfey, a fiend or a Great Old One. Each patron has their own set of benefits they bestow on the player. It’s clear that the Warlock is not as powerful in magic as a Wizard or a Sorcerer. The Warlock spell list is not as expansive as either of those two classes. The Warlock also uses different rules for spell slots. This means that a Warlock character will never have the ability to cast as many spells as a Wizard or Sorcerer.
But, there are benefits to building a Warlock instead of a Wizard or Sorcerer. Warlocks gain the ability to use Eldritch Invocations at 2nd Level. Players are also able to wear light armor as a Warlock, an option not normally available for magic users. Warlocks are also a d8 HD character build as opposed to the d6 HD used by the other magic users. If a player wants to build a more well-rounded magic user, the Warlock probably has a leg up over a Sorcerer or Wizard.
What is missing from the Player’s Handbook is the other side to the pact the Warlock has made with their patron. Each entity is going to have their own motivation for making a bargain with the character and their own demands from them. A fiend may have enlisted the player to serve in an army of the damned or just be collecting souls. A Great Old One may need the player to open a portal to allow them access to the game world. No matter what the goal of the patron is there needs to be a push for the player to complete their part instead of just advancing their own character goals. If a player builds a Warlock who has Cthulhu as a patron, old squid face isn’t going to sit idly by while the Warlock and his adventuring crew loot dungeons. The impact of the bargain needs to be felt in the game.
The first thing that needs to be established for a Warlock build is what the entity wants out of their deal with the player. The DM could let the player come up with this on their own when building the character, but there is always the risk that the player will determine that a Great Old One granted them power in exchange for unlimited apple pie. A player is very unlikely to choose a demand from the patron that has any real weight. The best idea would be for the DM and the player to work together to establish a bargain that is fair, but not ridiculous. Ideally the patron’s demand would be something that fits easily within the structure of the story the DM has created. A Warlock character should be able to move towards the goal while also operating within a party and advancing as a character. The demand should also not be easily satisfied by the player, but it should not be impossible either. The DM may want to use a goal with multiple parts so that the player can complete sections of it without completing the entire thing. A devious DM could even choose to not reveal the full details of the bargain to the player right away, instead providing clues throughout the adventure.
Once the demands of the patron have been established there needs to be a compulsion for the player to carry out these demands. This compulsion can be realized through a physical effect the player feels such as vivid nightmares or horrible pain that only subside when the player is working towards the goal. It could also come in the form of direct threats or punishment from the patron. A fiend may send demonic servants to attack a Warlock who is not keeping up their end of the bargain. Whatever the compulsion is it should increase in effect the longer the player ignores the entity’s demands. The type of patron that the player picks and the entity’s goal will also have some bearing on the level of patience the entity exhibits with the player not achieving the goal. An archfey who wants the player to hunt down a legendary white stag will have more patience than a fiend who is desperate to escape the underworld.
There is one optional thing that a DM might want to utilize with a Warlock character; this is marking the PC in some way as a result of their bargain with a patron. This will allow other characters who serve that entity and characters who oppose that entity to identify the Warlock if they know what to look for. Serving an archfey may cause the character’s eyes to shine bright green. Serving a Great Old One may produce a spot on the character’s hand that resembles the rune mark of that entity. If the DM chooses to mark the PC it should not be anything that would lower the Charisma of the Warlock.