Warlock

Warlocks are the rarest of the four major branches of the initiates (after mages, cultists and demihumans), which doesn't mean that they are an unpopular choice - it's just that it's a particularly hard to achieve choice. That, like all others, has its positives and negatives.
Warlocks are born out of the Gods desire to play their own games, similarly to how Powers play theirs. While they lack the sheer scope allowing them to do that on a multiverse-scale, they can do it on a much more personal level.
How does one becomes one of them? It requires an encounter (chance or not) with a demi-power. Then, you have to not die on the spot. If the god thinks that you might be useful (or you get it otherwise interested in you), it might offer you to become one of its warlocks. This is an option technically available to everyone that isn't a cultist or a demihuman (due to them already being claimed by something much bigger) - so, a mage. Or a very lucky/unlucky mundane.
There are three categories of warlocks which is based on their level.
Contract Stage warlocks are allowed to use the domain of their benefactor in a single way, with a well-specified activation mechanism. An example might be a warlock of a god of rats allowed to make a single target (that they need to point at and say an activation phrase) undergo a violent end akin to being devoured by a swarm of invisible rats. Which continues for as long as they can continue to point at the attack's target, with the effect continuing the longer the finger is pointed at it.
Those contracts might have a defined length or be permanent. Your ability to use the power of the Powers will be sealed for the duration, limiting you to using conventional weapons and using your new power. There are also cases for other demands, such as feeding some wild rats each time you use it. It all depends on the contract in question.
Authority Stage warlocks are allowed to use the whole scope of their benefactor's domain, in a much less restrained way. To follow the rats' comparison, an Authority Stage warlock will be able to command rats at a large area, see through their eyes and hear through their ears, perhaps execute a handful of magical techniques similar to the ones listed down next to the Contract Stage.
Authority Stage warlocks are something that's done for life. Your patron will always know where you are. It's almost impossible to get out of such agreement, unless your god gets slain. As a result, those are typically done by people who are emotionally invested into the god's domain.
Terminal Stage warlocks are an extreme rarity, something that's a double-edged sword (and that's putting it lightly). To become Terminal Stage warlock is to become practically merged with your patron. Widening the gods' understanding of the world and the human part of it, while giving the human a full scope of the gods' power. In exchange of being allowed to move around only as much as the god wanted it to.
This is, as stated, extremely rare, as it's not perfectly beneficial for either side.