Warning before post, this is my favorite new pokemon so expect bias. This mon has been doing wonders for me as a lead, maybe wonders is a bit of a stretch but it gets up rocks nigh-every time with some extra hp leftover for a will-o-wisp, eq, or protect to get some hp back. I have been playing around with a Runerigus that looks like this
Runerigus @ Rocky Helmet/leftovers
Ability: Wandering Spirit
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
IVs: 0 Atk
- Protect
- Night Shade
- Will-O-Wisp
- Stealth Rock
This specific one can ALWAYS take an icicle crash from a galarian-darmanitan, even banded
(calc for those curious: 252+ Atk Choice Band Darmanitan-Galar Icicle Crash vs. 252 HP / 252+ Def Runerigus: 270-318 (84.3 - 99.3%) -- guaranteed 2HKO yes that is adamant)
so you will almost always get rocks up unless some real demonic stuff is in front of you, having two immunities means that it can be a useful way to waste turns on dynamaxes(max lightning from dragapult being the main one off the top of my head). Overall I think the mon is solid but probably not OU worthy. I do think it will be viable in the tier as it is nearly un-OHKOable. Also he looks really cool.
why don't you just use body press instead of night shade... you have a max defense build.