What is a high level monk like?

[MaxWilson]

Generally, they have below average AC (but are usually OK defensively due to great mobility and strong saves), poor damage (few relevant attack feats, few relevant magic items, limited or no access to common damage boosters), and poor battlefield control (Stunning Strike can apply multiple times/round, but targets Constitution, which most things are proficient in at these levels, and Legendary Resists are also common).

When people say things like this I always wonder what kinds of encounters their DMs construct at high level. Not saying it's not true, just saying it's not inevitable, and there are plenty of high-level monster fights that monks are highly effective in.

How about a lich, a nalfeshnee, and a couple of air elementals? (Deadly x2 for a 20th level party.) A DC 19 monk has a 40% chance per hit of stunning the Lich (+10 Con), a 35% chance per hit of stunning the nalfeshnee (+11), and an 80% chance per hit of stunning an air elemental (+2). With up to five attacks per round (e.g. Flurry + Opportunist for a Shadow Monk), a monk has excellent odds of stunning the lich on any given round (will stun the lich 1.13 times on average without advantage on attacks, 1.69 times per round if it's already prone or stunned or in some other way giving the monk advantage) even if the lich Shields instead of Counterspelling. Or it could nullify both air elementals if the lich isn't in range. Even the Nalfeshnee, while not the best target, is certainly worth stunning and will get stunned 1.4 times per round on average. (74% chance of stunning the Nalfeshnee each round, even if the Monk doesn't have advantage, or Bless, etc.).

The monk is definitely pulling his weight in that fight even though the monsters have truesight.

What about a pair of death knights (Deadly for 20th level)? A DC 19 monk has a 65% chance of stunning a Death Knight with each hit. With Empty Body he can have advantage for an 84% chance to hit, times 65% chance per hit, equals 54% chance of attack of stunning the Death Knight. (1-0.54)^5 ~= 0.02, so there's only 2% chance for the Death Knight to avoid stunning. There's a pretty good chance the monk can stun both Death Knights every round for as long as his ki holds up.

3 Beholders is also a Deadly fight for level 20 PCs, and they're even more vulnerable to Stunning Strike than Death Knights: +4 to Con saves, AC 18.

Honestly the problem IMO with high-level monks is exactly the opposite: they have trouble with swarms of lower-CR monsters (if they're not Elemonks). A dozen Wraiths (Deadly for 4 level 20 PCs) will be a harder fight for four monks than 3 Beholders.

One of these days I'd like to run an all-monk party...