Evolution of the Mind Flayer

The Mind Flayer is one of those iconic D&D monster that players love to not ever have to deal with in their campaigns. Let's look at how it's changed over the years. For 2e AC and THAC0, I'm going to put the more modern d20 system values:

2nd Edition:

The 2nd Edition Monstrous Manual is about 70% fluff and 30% stats. The stats can be kind of confusing, and there's often stuff like "if you find this in a cave, there's a 25% chance it will have eggs." There's generally enough fluff to be clear on what kind of habitat you find the monsters in, what they'll be doing, what kind of social hierarchy they have, etc. Anyway, the 2e Mind Flayer is a creepy bastard who lives in an underground city centered around a huge brain and keeps a couple mind-controlled slaves around. The artwork is terrible.

Hit points: 40
AC:  15
To hit: +9
Special crap: Resists magic powers and 90% magic damage, can stun for 3d4 rounds, tries to carry off stunned creatures and EAT THEIR BRAINS!!! The Mind Flayer has a tentacle attack. If he attaches all four of his tentacles, he sucks out your brains and YOU DIE!!! Psionic Mind Flayers are in there as an option, but I won't get into that.
Magic spells: Suggestion, charm person, charm monster, ESP, levitate, astral projection, and plane shift.

3.5th Edition:

The 3.5 MM gives the Mind Flayer about two full pages, including some nice illustrations, several round-by-round paragraphs detailing its tactics, how to run the monster as a character race, and a more powerful variant.

The basic Mind Flayer is an 8th-level enemy:

Hit points: 44
AC:  15 regular, 12 touch, 13 flat-footed
To hit: +8
Special crap: Gets to do its regular attack four times as a "full attack," has flat resist 25 to magic, can do that same stun blast and 4-tentacles-and-you-die trick, 11 skills that I'm not going to list, has Improved Grab, has three feats that you have to look up if you don't know by heart (Improved Initiative, Combat Casting, Weapon Finesse).
Magic spells: Charm monster, detect thoughts, levitate, plane shift, suggestion.

There is a 17th-level version that is a bit more complicated, but one thing to note is it has 27 AC, in keeping with 3rd's +1/2 level scaling concept.

4th edition:
This is the basic 4e Mind Flayer. It starts at level 14, so its numbers are a lot higher (4e has 3.5-like shifting). It's stripped out all the spells, feats, and most of the skills and boiled the monster down to two special powers.  There's a 18th level version that has a bunch of powers, but I'll leave him out.

5th Edition:

The 5e MM has solid amounts of fluff and artwork. Each race typically gets a full page. The general feel is AD&D, but things are organized into tight, 4-e style stat blocks rather than 3.5's rambling text. The art is the best yet.

Hit points: 71
AC:  15
To hit: +7
Special crap: Advantage on saving throws against magic, and his save bonuses are pretty high. Not only is the mind blast back, but now it does 4d8+4 damage! He grapples you with a tentacle, and if you can't escape, he'll hit you next time with Extract Brain. Basically, the mind blast, grapple, and extract do so much damage that if you get hit with all three, you'll probably die.
Magic spells: Detect Thoughts, Levitate, Dominate Monster, Plane Shift.

WHAT IT ALL COMES DOWN TO:

While the numbers and powers look pretty similar, you have to remember that 8th-level 3rd edition Fighters will typically have an attack bonus of around +12/+7 (because having multiple attack bonuses is better), so 15 AC is basically a joke. In 2nd edition, the 8th-level Fighter attack bonus is between +7 and +9, and in 5th edition, it's the same. So again, we see that 5e is bringing the numbers more in line with AD&D.

One thing I noticed when reading the older stuff is just how much of a mess 3.5 is. The AD&D MM tends to have a serious problem with clarity and presentation, which the 3.5 MM solves with lengthy, detailed descriptions and just overall increasing the complexity of the system. Seriously, who wants to look up feats for monsters? Really, three different ACs? I try not to rag on 3.5 too much, but the 3.5 Monster Manual was a big step in the wrong direction.

The 3.5 die-hards are crying bitter tears over the loss of monsters as playable races and the reduced complexity that makes it too "casual-friendly." The fascinating thing is just how few of 3.5's monster-related innovations are retained in 5e. Essentially, 5e and 3.5 can be viewed as two entirely different revisions the AD&D source material. 3.5 is an across-the-board increase in precision, complexity and detail. The precision is welcome, but there are far too many additional rules, and the MM tends to be long-winded in explaining how to run a particular monster. 5e, by contrast, goes the opposite direction, making everything tighter, cleaner, and more to the point. It's heavier on the fluff than 4e without rambling the way 3.5 does. This is a running theme in 5e, and it's why I think it's the best D&D yet.