What level should my players be when they face mind flayers?

Question

I'm running an adventure for 3 level 2 (at this time) players. They've come to a town that is overtaxed and starving, and the government hasn't made a move. It turns out that most of the leaders are being controlled by Mind Infestors, small, squid-like creations of the Mind-Flayers. They latch onto the backs of people's heads and control their actions. I'm having the players learn more and more about the conspiracy each session, like episodes of a TV series. When the players finally unravel the mystery, I'll have them enter either the blacksmith shop or the government base, both entrances to the makeshift mind flayer colony. This is where my problems come up. The players will be level five by then. In the DMG (while describing players level 5-10) it states the following:

These adventurers venture into fearsome wilds and ancient ruins, where they confront... crafty mind flayers...

My question is this: Are mind flayers on the higher side of levels 1-10, or the lower side? If I can't use them, I'll probably upgrade a commoner so it would be a good enemy to stuff the majority of the lair and/or be an enemy on a random encounter table. Then I could have a mind flayer boss at the end of the dungeon and promise more of them later.

Answer 1

Towards the high side

This is all detailed in the Creating a Combat Encounter section of the DMG starting on p.81.

The most relevant parts are the sidebar on p.82:

When putting together an encounter or adventure, especially at lower levels, exercise caution when using monsters whose challenge rating is higher than the party's average level. Such a creature might deal enough damage with a single action to take out adventurers of a lower level.

The Mind Flayer is a CR 7 monster - be careful in using it against a party with 6 or fewer average levels.

The rest of the information tells you how to work out how many creatures make an Easy, Medium, Hard or Deadly encounter. Despite the seeming preciseness of the math these are, at best, guesstimates.

The difficulty of an encounter relies very heavily on the skill of the DM in using the monsters, the skill of the players in using their PCs, the match (or mismatch) of the strengths and weaknesses of the particular monsters and the particular PCs, the overall situation and the availability of resources to the PCs (a trivial encounter immediately after a long rest may be a TPK immediately before one).

With that caution in place, for a party of 3 equal level PCs, and working the math backwards:

• 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 for 11th, Medium for 12-15th and Easy for 16-20th
• 3 Mind Flayers (2,900 x 3 x 2 = 17,400XP) is a Deadly encounter for 7-14th, Hard for 15-16th, Medium for 17-20th
• 4 Mind Flayers (2,900 x 4 x 2 = 23,200XP) is a Deadly encounter for 7-16th, Hard for 17-19th, Medium for 20th
• and so on ...

Note that this is the per encounter rate - you also need to consider how many encounters the party can endure before needing a long rest and where in that mix they will need to slot in a short rest. If the party will have one titanic battle then it can be a harder encounter than if they need to get through 6-8 in a day.

Answer 2

Technically, you're covered with the rules from DMG about encounter building.

Nevertheless, Mind Flayers can be extremely deadly to even high-level parties if most characters have low intelligence because of the mindblast and the insta-kill attack, so tread carefully.
A possibility you have is to have some of these abilities negated by some story elements. For instance:

• special blessing from some ancient force, getting the favor could be a quest in itself

• kryptonite-like stone cavern that prevent the mind-flayer from using mindblast, finding out about this and luring the mindflayer there could also be interesting

• etc...

If you do that, you could even have your low-level party face the beast (they're not very resistant) and deal with the aftermath (mindflayer community bearing a grudge against the party?)