Unfortunately it looks like you hit the 1500 turn limit which is an auto loss :(

I assume that Ultimate Death Knight you were facing was healing itself?

Yes they both where 

Hi there! So yeah, once you hit the turn limit as an attacker you are considered to have "lost" the match. The game is saying if you can't kill a defender in 1500 turns, you likely never will.

I just had the same issue in an arena match. For well over 5 min DK showed no HP yet he kept coming I saw him take at least 50k damage after he showed 0. I had 4 lvl 60 heros 1 was fully asended. How can this be possible. He did not show no kill or anything like that. I am truly baffled so 10 min in I just quit as he wasn't going to drop.

I'm fairly certain this is a visual bug, usually based off of an interaction with Ronda. If you have more issues with this or it's something diferent, feel free to start up a thread in https://plarium.com/forum/en/raid-shadow-legends/677_bug-reports/

UDK heals from his passive, when an enemy heals. Deliana is one of the worst champs you can bring to fight UDK. She deals low damage and will heal the opponent.

sums up arena silver eh?  Ultimate troll knight.  

Also i'm still trying to figure out how so many UDKs i've fought won't die despite having no visible health bar and no unkillable.  Or how one lvl 24 UDK healed for 2 procs of 8000 each in one turn, WITHOUT continuous healing buff, and none of my champions were healers  (that match, UDK healed for 72,000).  It's seriously frustrating how some just won't die.

There's a known visual bug with UDK, where his bar will show as 0 HP, when he actually does have HP.

He also is often equipped with Regeneration artifacts giving him +15% HP healing per turn.