Beholders are stronger, MUCH stronger, Beholders get 10 spell like abilities (no more than 3 per 90 degree angle) every round. The abilities are... Charm Person Charm Monster Sleep Flesh to Stone Disintegrate Fear Slow Inflict Moderate Wounds Finger of Death Telekinesis As well as that they have 360 degree field of vision, and can't be flanked, can also open (or close) it's central eye, which (when open) produces an antimagic cone, directly in front of it. All spell like abilities are cast as a 13th level sorcerer.
WotC - WotC D&D Monster Sources Beholder variants "Physical differences are the most minor of variations among beholders. Two notable beholder variants, the elder orb and the hive mother, possess wildly different abilities from those of normal beholders. Beyond these are the myriad forms of beholderkin, true aberrations among the race. Creatures like gauths, overseers, and eyes of the deep are the subjects of wrath among true beholders, for they are similar enough in appearance and ability to be regarded as mockeries of the beholder self-image." "Eye of the Deep: An eye of the deep rarely comes into conflict with true beholders, for this aquatic variant dwells deep underwater. It has only two eyestalks, but its massive pinchers make it a dangerous combatant." Beholder - Monster Manual [p.25] Aberration, Large / CR:13 Eye of the Deep - Lords of Madness [p.138] Aberration, Large (Aquatic) / CR:8
The MM lists it as having 60 HP (average), I'd suggest wearing a cloak of resistance with as high a bonus as you can afford, and really good Willpower and Fortitude saving throw bonuses (I think Telekinesis is the only spell it has that requires a Reflex save). Virtually no chance of doing sneak damage on this thing (save being invisible and sneaking up behind it - a frontal approach would see your invis rendered useless in the antimagic cone). Ooo and I beleive a LOT of luck would come in handy too.
The Eyes of the Deep in 5.0 aren't true Eyes of the Deep (well, neither are the original ones in the game) and are more of a lesser beholder. Regular old Eyes of the Deep mostly just have a stunning gaze and then rip you apart with their claws. In my test games, that version was pretty boring to fight and also easier than what already existed in the game, so I went back to Troika's concept, assumed it was a new monster, and tweaked it to be rules-compliant. Well, the idea is it first creates an Illusion of what the target fears most. The Will save is to disbelieve the illusion. If that succeeds, the spell ends. If it fails, then you get a Fort save to not die as the illusion "kills" you. If that succeeds, you only take a handful of damage. If it fails, you die.