
TheGreatWot |

Uh... all I can see is that he's a 20th level Large sized brawler. That's a CR 19 threat, and not really a difficult encounter for a party of 20th level PCs- especially 6 of them. Also, Thanos strikes me more as a LE kind of guy. Very methodical, even honorable. But also very evil.
If you want to challenge a large party of 20th level PCs, you're gonna need multiple threats. Action economy is always on their side until you have as many actions as they do. Maybe make Thanos a unique creature instead of a 20th level brawler, so that you can add special abilities wherever you want. He could be a monstrous humanoid or outsider or giant.

Meirril |
Personally I'd be tempted to make Thanos a Titan, since he is known as the Mad Titan. But Titans are all colossal sized creatures. Though if you did you a titan, you could just throw it as is against the party. Most titans have a CR above 20.
A Genie isn't a bad decision for a base race. While I'd like to pick a Djinni because they are normally blue, that whole mist thing instead of feat ruins the image. Thanos is suppose to be tough, so lets go with a Shaitan and make it as tough as possible. Pasha is a Shaitan Nobel.
Now instead of a brawler make him a level 20 wizard. That will raise his DC from 13 to 19. Probably 20 because of special features beyond what a wizard would normally get (like all those d10s of hp). I'd slap on another 7d10+35hp for the extra 20 levels of wizard advancement. Or give him a full 20d6+100hp. He has to stand against a whole party after all.
Now create that artifact on his hand. First thing I'd do is let him use his wishes as he wills. That is 3 free wishes per day. Then give it the effect of quickening 3 spells like a greater metamagic rod. Free teleport at a move action. Let him cast and punch as a standard action to deliver any touch spell. Make it a +10 weapon. +5 to hit, another +5 in other bonuses.
Pump his stats. +6 to everything as an innate bonus (wishes). +6 enhancement bonus (appropriate gear). Raise his Int another 5 because advancement. +5 resistance bonus to saves (wishes). +6 natural armor and deflection bonuses (wishes). +7 shield bonus to AC (invisible force shield, wishes again).
And dump a lot of consumable defensive gear on him. That delaying doom ring. Scarab that blocks death effects. Poison immunity necklace. Defensive spells a plenty (made permanent via wish). Go wild.
And he wishes himself to full when he's below half, and plane shifts away when he is out of wishes. Have fun.

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I've just had a quick glance and the first things that leap out are that his AC and saves (particularly Will) are waaaaaaay too low for a CR 20 threat. Compare to other CR 20 creatures in the bestiaries to see roughly where these should be and work out how to get them there. Also Power Attack should be -6/+12 not -5/+10.