What abilities should a CR 20 monster have?

[Count Arioch the 28th]

I'm brainstorming a high level monster, but I don't want to make the mistake WotC makes and create something that a group of 5th level PCs can beat with a flay spell.

I was thinking:

Fly speed
Ranged attack of some sort
Ability to nullify hostile magic affects, preferably as a swift action
Some sort of protection against instant death effects

Any other points I should keep in mind?

[RandomCasualty]

Teleportation. You can't dispel a forcecage. Doesn't have to be long teleportation, it can just be tactical teleport, but it needs to have some means of teleport or access to a disintegrate spell.

Also what rules are you using? Like is shivering touch in the game? Is wraithstrike? That sort of thing.

[Surgo]

Should have some ability to take multiple targets out of the combat.

[Talisman]

Multiple frikkin' attacks. I don't know about being able to take out multiple PCs, but nothing's lamer than seeing the Great Evil Darkmungus take one action...the PCs each take one or two...and the Darkmungus is wolfpacked to death in two rounds.

Invulnerability (or at least, high resistance) to mental attacks.

A mountain of hp and some serious fast healing/regenerate (not 5...more like 20+). Enough so it can take average damage from the whole team for 3+ rounds without dying.

A few thematically appropriate abilities that can be used as a swift/free/immediate action. You don't need a list of 20+ SLAs, just a couple that fit well with the critter's theme.

Enough Int to be crafty and tactical.

Minions. Either summoned or dominated or bribed with sweet, sweet candy. The minions should be weak enough for a PC to take out in maybe 2 rounds - one with luck or high-level resources - and mostly exist to clog up the battlefield, provide tactical options, and prevent the PCs from dogpiling the Darkmungus immediately.

[JonSetanta]

Yep, emphasis on the minions. Abilities to communicate with hordes of followers at once, as well as deceive/control/eliminate entire armies of foes at once.

[Username17]

Using up resources from a 20th level party is hard. Hit points are basically meaningless and a 19th level wizard can drop a lot of whup ass in a day. There are two main methodologies:

Huge defenses.
Huge Attacks.

A monster which goes first and has a serious chance of killing one or more PCs in a round is a threat. It can just die when attacked. A monster which soaks up several Banshee Wails before it goes down has drained the party whether it gets a hit in or not.

Properly played high level characters are immune to most things. If you don't have mindblank from a spell or ring by 20th level you're a god damned idiot. Most high end characters will have death ward up for their entire five minute work day.

So imagine for the moment that you had a high level Beholder. It can either be in antimagic or not, so it gets to go. It lets loose with 10 high end attacks. Banshee, Weird, Mass Charm, Meteor Swarm, Prismatic Spray, Implosion, Poly Any, Disintegration Sphere, Forcecage, and Flensing.

That ought to do it.

-Username17