Edit: Based on my testing everything that is at least partially dodged or blocked is going to return damage when hitting Rammus W. The question about how much damage will be returned is still opened when on-hit effectiveness modifiers or multipliers are involved.
I understand that champions, minions or monsters that attack Rammus will be hurt by his W. But some champions have confusing auto attack interactions or abilities that makes me wonder if I will get shredded by the ability for no damage to be returned at all or if I will be the one shredding the enemy.
1 - Abilities with on hit damage modifiers
I understand those abilities will return the shield damage, but will Heroic Swing , for example, return the full damage every time Rammus is hit? Or just 25% because of the reduced effectiveness?
2 - Basic Attacks that applies spell effects
If the AA deals both spell damage and basic attack damage, W will damage the attacker. But the wiki says both for Akali and Khazix examples that they deal only ability damage, which would suggest they wouldn't be damaged. I don't think that is the case, right? Also why is Zeri not here? I though her autos are supposed to deal spell damage at all times.
Edit: Here is where Zeri's AA are listed. Does this mean every thing on this list ignores Rammus W damage and the majority of the examples n the list above do not?
3 - Pets that have different AA rules
If the pet behaves like a minion, like Naafiri or Yorick, it would make sense that they are hit back, but what if the pets deal 100% magic damage, like Elize, Heimer, Malzahar or Zyra? Will they still be hit back and I can expect to kill them in about 3 hits of my W? Or they will live unharmed and I will be damaged for no reason?
What about Daisy triggering Ivern's on-hit? Does she take the damage because she is the one attacking? Does the damage procs twice on her, fisrt because of her AA and second because of Ivern on-hit? Or does it return directly to Ivern considering she is a ultimate that triggers on hit effects, much like Katarinas R?
The Ivern case is similar to Shaco, Le Blank and Wukong clones. But giving clones are separated champions I would expect the damage to be returned to them and not their real counterparts.
4 - Why aren't there any videos testing these things?