This is how I see it.
Priests like clerics, druids, paladins, and possible other priests like shamans, oracles, and invokers Channel magic from their god, pantheon, nature, oaths, or other primal or divine being, object, or statement which has power to affect the Weave/Aether/Fade/Winds/Wild/Channel/Mana/Well.
These power are literally power channeled into your PC's soul and mind. At level 1 and 2, your soul is too weak to handle a full channel and you get mostly vanilla power.
Same with warlocks. A warlock's pact is a bastardization of a priest's channel. It is done by a powerful being who links you directly to the Weave/Aether/Fade/Winds/Wild/Channel/Mana/Well. Like a priest, your body cannot handle a full pact at level 1 and 2.
Look at Warhammer lore of what happens when a demon or old god blesses you too much before you can handle it.
That should inspire a new monster. The warlock who begs for too much power and becomes a mindless eldritch blasting mutated horror.