If you are new to Raid Shadow Legends you could be tricked into thinking you just pulled an awesome Undead champion. Dont worry I got you, this dude SUCKS!
Rotting Mage is a Spirit affinity rare from the Undead faction and is widely considered to be rotting trash. Do not invest books or masteries on this champion.
He does have a good chance to stun on his A3 that can be used early game to help through some of the faction wars content, I would never take him past level 30 or give him gear above level 8 even for this task.