I'm not sorry for saying this: That ult, as it is, seems like a bad concept. Letting a tanky mage take someone out of battle, then 1v1 someone for that long isn't logistically survivable by the opponent, as surviving means not having enough health to counter and having all your abilities on CD. The opponent is punished for surviving. How is that fun? And then afterwards Morde pulls/can pull something akin to a Nasus Ult AOE larger than Nasus' own. Seeing this rework, it's no longer a wonder that there's so much power creep in this game.
That needs a hard nerf: Reduce the time by .25 seconds, and let him take partial damage (let's say 10%) from other champs while he's in there.
Alternatively, don't let him heal while inside. He's a tank, he can and should have to survive his own ult without healing if it's that powerful, especially since it's a move used when one is on the offensive. Being able to hit the delete key on nearly any champ like that is game breaking as well.
Also, getting pulled into an alternate dimension and getting thrown back is enough of a distraction to the player that it should be counted as part of Morde's effectiveness. In teamfights alone, the confusion of having to stop and look around to find who you're supposed to focus (assuming you miraculously take no damage) is disorientating and can cost a death.
As a secondary question, after seeing all these infuriatingly OP kits come out of PBE for years, is it just me or does PBE need to be restructured in a way that exposes these issues before the kits officially go live?
Maybe I'm just salty, but after doing fairly well in game, and then losing the game because someone on the other team decided to play the all-in-one tank/mage/assassin champ, I feel the urge to call attention to this. It's not quite as broken as the Rengar rework (From experience: 5 Viegars, two of them with 800+ stacks, needed to bring a skilled one down in late game)... but it's close enough.