I'll go with Vothsair Reaver, though I adore Ireguard equally. Favorite spell; Retribution, super diverse and flavorful. For the Relic, it's actually gonna have to be Nefari Exchange. I just like the random heal in UD. And Demon Shield for the spot Firk you.
Korona, Bok Huntress - While I miss the old version of her, especially oblivion shield pre-revamp, she is still a lot of fun. You can go high mobility in bleed decks, or just make her into a sustainable UD battle tank, stereotypical UD style with lifedrinker and thirst for battle. She looks like a leader ready to charge into battle and show everyone how it's done. Honorable mentions to Vothsair Reaver and Hekatian Arbiter. Reaver is just like, nah you ain't getting by me. Arbiter is just a scary looking beater (but not that scary post-nerf or whatever happened to her).
Minotaur Mageknight - artwork is incredible! He utterly wrecks large champ face. He's a ranged unit who'll eat you in one bite if you get close. You feel like you're putting something that matters down once you see his sprite! Summon trap spider - getting to watch people move every unit 1 square at a time. Its like the mageknight has a spot the exact right size to fit one of these guys inside its satchel. Lava front - 4hp gas station for fire eater units comes with 4 fuel pumps, fixes perseverance, and 4hp damage to enemy units! Contests a font too. Helm of chaos - champs with leech vitality or adaptive or soul siphon.
UD has some of the best artworks and badass play feel out of all the factions! That's what got me hooked on to UD from the start. All hail the Underdepths!
