Mutating the Mind Flayer
The term Mind Flayer is not in the SRD, it is the intellectual property of Wizards of the Coast. Which means nothing unless you want to publish your version professionally outside of the Dungeon Master's Guild.
The 13th Age RPG calls its squid-head the Soul Flenser. I'll call my version the Tarsiss, in case this ends up in a future project.
Besides the mind flayer, the khepri: bugs that lived on human bodies from Perdido Street Station and of course, Mi-go brain cylinders.
Everyone, from D&D players to Stranger Things fans, loves a mind flayer. Here's the first mind flayer I ever saw, from the original Dungeons and Dragons white box supplement II Blackmoor by Dave Arneson. 1975!
Let's mix it up, creating a new creature, inspired by the mind flayer -- one that players won't know all about. For this example, let's use the Cypher System, as stat blocks are easy. If we like it we can make a 5th Edition or a 13th Age version. (Edit: the 13th Age version is here.)
What do I like about mind flayers I want to keep?
- Squid head, humanoid body
- Super smart
- Mental attack
- Eats brains
I love the fact that these creatures have humanoid bodies and squid heads. My creatures (the Tarsiss) are squidlings that take a human body, cut off the useless head, and replace the head with their own squid like body. They are super smart, use bizarre technology, and are practiced in dimensional travel. When not using weird tech weapons, they have a psychic attack.
And of course, they eat brains. They don't mind canned food, so they keep humanoid brains alive in weird devices that look like fat thermoses with ports for connecting to other weird devices. Captives in these brain-jars can be made to run machinery. Imagine the fun when the players find a Tarsiss outpost staffed by human brains in jars!
Level 7 (21)
Armor 2 (force field, works against Intellect attacks as well)
Lightning gun: 7 damage, long range, (depletion 1 in 1d6) level 7 artifact
Psychic blast: Short range Intellect attack that does 7 damage (ignores armor) and causes target to make a Might Defense roll or take 5 Speed damage.
GM Intrusion: The tarsiss attacks up to five targets within short range with its psychic blast attack.
Danger: This creature is more dangerous than a typical 7th level creature. That's the way we like it. Tone it down if you and your players prefer easy fights.