Harbinger

Harbingers would look like strays, but with red eyes and black 'robes'. Instead of a bow, they would carry a twinblade, which they would throw if the player is further than 5 blocks away, and use as a melee if they are 5 blocks or closer. They would only spawn at the top of Summoner Towers or in mausoleums. If killed they would drop 1-3 summoner crystals, 0-2 deathwings and a harbinger head, as well as a chance to drop their twinblade. They would be able to fly similar to blazes, and would be hostile to all non-undead entities. They would have initially 16HP (8 hearts)

There would be nether- and end-variants for Harbingers. Nether-variant Harbingers would be unaffected by fire damage and have orange eyes. They would have 24HP (12 hearts). End-variant Harbingers would have purple eyes and would slowly regenerate health, 1HP every 5 seconds. They would have 30HP (15 hearts).

A harbinger head could be combined with three summoner crystals to craft a Reaper Stone. Deathwings could be combined with an elytra, which would create a winged tunic, offering some protection as well as the flying ability.

Edit: I am aware that the developers have said that no such item as a winged tunic will exist, but I will leave the idea here, until someone else thinks of a better use for deathwings