You didnt consider speed. Filling other champs meter instantly even by just a few % can make or break a win in arena.why do you think ppl use gorgorab or apothecary in arena so often?

Lets say your kael is fully stuffed with dmg but has just moderate speed.

Lets say kael has 90 speed and enough dmg to wipe at least half of your enemy team.

Say your diabolist has 101 speed.

The fastest enemy hero is athel, she sits at 95 speed and can wipe ur team if she goes first what would be the case if you hadnt your diabolist in your team.

Now your diabolist goes first at 101 speed, fills your kaels meter from 90% to 105% which allows him to go before the enemy athel. Due to his dmg and crit luck he kills the entire enemy team with his aoe.

Speed and filling turn meters is HUGE in arena. Who goes first has a big advantage. At the same time reducing enemy meters is a big thing too. Stuff your dia with speed + accuracy and ur good to go.

Its mostly a PvP skill. Used for mid-slow but heavy atk geared champions. Diabolist in full speed set gives to your entire team powerfull speed boost and a decently good opportunity to play first before enemy champions. Further actions depeds from your tactic - stuns, counterattacks, taunts or just a wave of powerful AoE from 3 champions will burn out most of midcore teams.

I get what both of you are saying, but I feel like it would make FAR more sense to have another champion in there that can do damage as well, rather than someone who can just speed up the other champions.

Like say she gets Kael and Aothar to attack first.  Great, but then say neither of them kill any opponents.  Now, Diabolist is just a waste of space, as she did no attack her turn, did no damage, yet took up a spot on my team.  In that regard, I feel like she can be a very risky gamble, where if you don't succeed with first strike, her on your team will put you at a disadvantage.

I mean if you have some really, really strong glass cannons and think they can one-shot in the first turn, then I guess she could be worth it.  But that is the only situation where I see her viable over filling that spot with another champion.  Thanks for the feedback though.  I do see that in some situations, she could definitely be useful.  Since Deathless is my ace-in-the-hole though, makes absolutely no sense for me personally, especially since my Nethril usually gets first strike and knocks down all the turn meters of the opponents by 75%, not 15%. ;)