I like venthyr alot, both mechanically and thematically (My favorite archetype in magic is black/white vampires and venthyr paladin nails that hard.). The cooldown is long and awkward but when ashen hallow is out you feel powerful. For pvp it might not be the best covenant, but Ashen hallow and door of shadows both being shadow school has it’s perks. Plus it’s always fun to fight someone who doesn’t know not to stand in the big red death zone or to drop it on top of a big battle/flag.
Not to mention door of shadows doesn’t seem very limited in the ways you can use it for vertical mobility. As long as you can see a place, it works, and more often than not the very edge of a ledge counts as on top of the ledge.