Today's point on the 30 Day Challenge list asks me to pick a favorite Immortal or Outsider, and I think the most solid choice one could make here is the Marut. As the Monster Manual says, the Marut (see Inevitable) represents the certainty of death, which can put them in opposition with PCs a number of ways.
The Marut may come after the party after they drink potions that may extend their lives, or if they abuse the recurrence of life that spells such as raise dead or resurrection bring. Otherwise, they may run afoul of the Inevitable as it tries to bring an end to the party's enemy, like a lich who has cheated death for centuries through its undead state, or a powerful cult who sacrifice virgins and bathe in their blood to prolong their life.
Not only is their mission dire, but the Marut are completely unflagging. As constructs, they never need to rest, and would search planes for eons before they find their quarry...assuming one lived that long.
This is not to mention that they just look ready to bring eternal rest to those who seek to cheat death. From their fists, crackling with powerful energy to their elaborate bronze armor covering their jet black bodies, the question of if these enforcers of universal truths come in peace or seeking to mete punishment is moot.