I'll say that I don't think Vecna's reactions should allow saving throws. The power of high level monsters is that they should have one or two moves that are just "They can do that." It adds to the challenge IMO at higher level when you have to plan around not DODGING big damage, but instead anticipating and RESPONDING to big damage.
Sure, and I'm ok with that.
But look at what those two actions do to gameplay.
The counterspell means that you cannot cast a spell while within 1000' with line of effect with Vecna.
The knockback means that only your first attack can hit in melee range. If you lack 60'+ of additional movement, you cannot stand up and close the distance to get your 2nd attack in.
Both of them are
action denial with no save.
Meanwhile, an archer? Unaffected. A spellcaster who buffs as opposed to direct damage? Also relatively unaffected.
There isn't an active puzzle to solve for the most part. Either your build is unaffected, or nearly completely nerfed.
And at 3 reactions/round, the only choice is "who does Vecna screw over this turn" as the DM.
I am ok with screwing over PCs, but I think there should be counterplay! Hence my attempt with the design of the Hand and Gaze abilities. The Hand really screws over melee PCs, but there is counterplay - you can boost your AC, you can get ways to escape grapple like freedom of movement, you can even just deal 100 damage to it and escape. The eye, imposing disadvantage and preventing hiding/cover, is also pretty nasty; but the counterplay is you can attack the eye (you
always have full line of effect), and it is one target.
The counterspell doing a concentration save ends up rewarding that character who got war magic and con saves to make their saves super reliable - but at like 24ish DC, with the eye cancelling out war magic, the counterspells are still gonna get through unless you protect yourself with (say) a paladin and an artificer. And if we have a epic tier paladin, artificer, war magic specialized spellcaster all together required to get through Vecna's counterspells, that is sufficiently epic for Vecna.
TL;DR: Have "no save" and "auto hit" abilities, just don't make them also "oh, you don't get to do anything because I chose you, and there is no counterplay possible".
