The 5e warlock patron and its little running messenger

Poor Cartero
(postman from The Legend of Zelda)

In an upcoming 5e game, I'm playing a tiny warlock named Merla.

And like all warlocks, Merla needs an otherworldly patron (page 107 in the PHB). I haven't decided on which one, but it'll probably be some sort of fiend.

According to the PHB, the patron whom you struck this pact with, grants you these powers in exchange for favours you need to carry out (burn this village of peaceful rabbits, tease your sister, eat a bowl of maggots under one minute, and so on).

This is what I'm thinking for Merla's fiend patron:

• It's the loneliest fiend in the world
• It communicates with Merla through handwritten letters, delivered by the always-on-the-run-always-out-of-breath maildaemon Cartero MMMDCLXVI
• When the fiend grows tired of a warlock, it will try to lure him/her to come visit it in its extremely lonely castle - only to kill and turn the warlock into the next Cartero (that is, Cartero MMMDCLXVII)
• The fields surrounding the castle is crowded with former Carteros. Since their services are no longer needed, they've built small mailboxes out of whatever (bones, stones, almost dead birds), and spend all day delivering stuff (junk) to each other
• The fiend "dabbles in poetry", so bad that the paper it writes them on turns into horrible, soul tormenting spells