Aite. So thoughts -
There are basically two champs in the game that play well in go-second vs go-second matchups - IE, where you're basically in a stalemate where neither team can win. These are scenarios like where you're against Siphi + Duchess + Ursaga + Candy. Your go-second team of, say, Mithrala + Brogni + Raglin + [Someone] is looking for a person to actually be your win-condition. Their team won't win because you can just keep reviving your team with Raglin's super-low revive, and the fact that their nuker is going to repeatedly target Brogni due to weak affinity. But your team won't win if you are running a "traditional" go-second DPS like, say, Ragash, because they're too tanky and will just keep reviving themselves over and over.
The aforementioned two champs are Mortu and Rotos. Mortu because Peril is basically going to oneshot literally anyone when it eventually procs, and your game is to just sit there and wait until it does. Rotos because, even though he's going to die over and over, eventually, you'll get a revive on him and he'll get a turn, and eventually that'll translate into his A2 stealing some HP. Then he'll keep dying again and again, and eventually you'll get another A2. After enough of those cycles, you'll have him strong enough to not die, at which point you get his A2 faster, and then eventually his A3 blocks revive and you win.
Fortus is similar to Rotos in that respect, but given he's DEF-based, he won't be dying nearly as often. In that aforementioned Mithrala + Brogni + Raglin team, Fortus fits perfectly. He'll slowly get stronger as the fight goes on - while initially his A3 will do very little, eventually it'll scale so much that he'll just oneshot the other team.
Will it be a slog? Absolutely. Especially against teams with block debuffs, his fear won't be activating nearly as often as you'd like. But, it will eventually land. And each time it lands, his A3 gets stronger.
All that said - he's absolutely worthless outside arena. However, if I didn't have both Mortu and Rotos already, I'd 100% be pulling for him, because go-second teams without a reliable win condition just don't work.