Papalith is an old, crotchety Necron Monolith. He has been around so long, that even other Necrons call him old. He previously fought in a Third Edition Necron Codex army, before Necrons got nerfed. He still remembers it, and is just as sore about it as he was when he first got the news. He can still kick some major tuchas, and enjoys still being the most unkillable tank of the game (outside of Apocalypse, but Papalith doesn't care for that either).
Not senile like Grandpa Dreadnought, but can never find the blasted keys to his tombship. In his younger days he single-handedly installed the Pylon sprinkler system that surrounds his home tomb. Keeps them young races off his perfectly groomed Tombworld!
Notably, he is far older than Grandpa Dreadnought or Bjorn the Fell-Handed, being a Necron and all. Not that either one actually cares. Papalith also has a tendency to mutter about "those new, young Monoliths these days."
He also has an awesome beard. And he is still the only vehicle in the galaxy to have grown facial hair.
And then Matt Ward came along...
And Papalith barfed four-score and twenty Flayed Ones at him. It should be noted that despite any and all things in the new Codex, Papalith still takes no crap from Power Fists, because he adheres to the spirit of the rules, not the word of the rules. 3rd Ed rules, to be precise; and nobody dares tell him different.