Another type of abberation I think would make a good patron is a Mindflayer Elder Brain, the warlock has a Mindflayer tadpole inside them and the elder brain forces them to work as a scout and spy or the tadpole is activated and turns the warlock into a mindflayer, where ever the warlock goes, mind flayer slavers follow in there wake. Illithocyte: Illithid tadpoles that survived the fall of a mind flayer empire, they evolved into a new life form and now crawl about in groups seeking psychic radiation on which to feed. Then WOTC releases this little nugget.

When an Illithid tadpole grows up the correct way through ceremorphosis, or the insertion into a brain, they grow into proper Mind Flayers. While it may seem like a sentient brain, the Elder Brain is one of the strongest creatures in Dungeons and Dragons. Each creature in that area must succeed on a DC 15 Intelligence saving throw or take 22 (4d8+4) psychic damage and be stunned for 1 minute. In the case of the tortithid the tadpole was implanted into a female tortle carrying eggs before laying them. It is highly intelligent, and feared for its powerful psionic ability. A creature can repeat the saving throw at the end … I say it’s progressing slowly because she’s so dumb, but really it progresses at the speed of the plot. Under normal circumstances, an illithid/mind flayer will reproduce by implanting a tadpole-esque offspring into a victim's skull.
Now we already know that Larian and Wizards are working together to rework aspects of 5e (see the cluster fuck that is the Ranger), we also know that all player characters in BG3 will have mindflayer tadpoles in their head, which grant them special psionic abilities. 1 Mind Flayer (2,900 XP) is a Medium encounter for 7-8th level and Easy for 9-11th 2 Mind Flayers (2,900 x 2 x 1.5 = 8,700XP) is a Deadly encounter for 7-10th, Hard …

If you want it to be something that happens over hours, it’s hour. In the final stage of its life cycle, a Mind Flayer may become an Elder Brain. [27] Kezreth : A living troop transport and battle platform created from the severed head of a shamed illithid. After this, “morphological transformations” occurred and after a week a new illithid was created. An innovative Dungeons & Dragons race gives players the chance to build a character infected with a mind flayer's tadpole. A mind flayer stands around the same height as a human, typically between 5'4" and 6'2", having humanoid shape and narrow build. Unimplanted tadpoles must be killed, because if they’re left to their own devices, the tadpoles will grow out of control and dumbly devour every living thing around them, including other mind flayer tadpoles.

One of my players has been dealing with her mind flayer tadpole for months of game time. The illithid, commonly known as the mind flayer, is an aberrant humanoid best recognized by its tentacled mouth, which it uses to suck out and consume humanoid brains. The tadpole ate away the victim’s brain matter and essentially replaced the brain, erasing all of the subject’s personality and memory, but leaving the physical body alive and under the tadpole’s control.
The mind flayer magically emits psychic energy in a 60-foot cone. Neothelids are products of mind flayer reproduction gone awry. Mind flayers reproduce by hatching thousands of tadpoles and implanting as many as they can in the brains of living hosts. If you want it to be months, it’s months.

The multiplication of mind flayer colonies happens when a tadpole, quite rarely, through ceremorphosis, creates a more powerful form known as a ulitharid (meaning "noble devourer" in Undercommon), who is biologically bigger, stronger, and more powerful and cunning than regular mind flayers.