how will this new item interact with clone/pets such as Yorik ult, Morde ult, or Shaco Clone...or Elise's spiderlings? As in, if the cloned champ has Tiamat/Ravenous, will the clone/pet itself proc its effect with its auto-attacks?
Also, a little cheating here, but looking at LoL wiki's list of on-hit effect related abilities, will this new item interact as imagined with: Master Yi's passive, Fizz's Urchin Strike (Q), and Irelia's ult (especially curious with this one)?
There are more instances, but those have been answered already (WW, GP, Rene, Shyv)
Also, continuing on with abilities that splash already...will it stack with Diana's passive and Nocturne's passive? (Malph's already answered for)
Another thing is how will it interact with Cho's passive spikes, Udyr's phoenix form, Morde's Q, and Voli's thunder claws?
It's an on-hit effect, which follows our standard on-hit effect rules.
So Fizz, yes.
Master Yi, yes, twice.
Morde's Q, yes.
Morde's Ghosts, yes.
Elise's Spiders, no.
Spikes / Phoenix / Thunder Claws - no - but these character's basic attacks
will apply the tiamat cleave, so they will work with each other, it's just that the magic damage splash attack won't trigger tiamat.
Tiamat/Hydra and Sheen/Trinity Force/Lich Bane procs are not going to spread to to the 'cleaved' targets, just the primary target for that individual proc, right?
Just the primary target, yes. Sheen + Tiamat is worse now, unfortunately.
Does the lifesteal work on all cleave dmg dealt by hydra? And does it factore in all your life steal?
Does it factor in all your life steal - Currently, no. We're tweaking the ratio of this - but right now it uses 50% of your life steal for the AoE portion of it - but we're still tuning.