SCENARIO: Purple team Sejuani uses Artic Assault (Q) and collides with the Blue team Zac who is mid-'Elastic Slingshot' (E).
If Sejuani has some type of spell shield will only Zac crash** or neither of them, or would they still bash heads (well head and… goo, I guess…)? What if both of them have a spell shield?
**When I say 'crash', I mean, stopping in their paths once they collide, like how it usually happens.
Best Answer
What many people don't know: Spellshields can block multiple abilities if they hit at exactly the same time. I don't know if this is intended or a bug but it is a fact.
Since Spellshields negate all the effects of a Spell (CC and damage), if one of them gets hit by a spell he will just pass through as if nothing has ever happened.
Scenarios:
Zac without spellshield is jumps over charging Sejuani with spellshield (your question)
Here both of them will "crash". Sejuani will stop since she hit a unit and Zac won't be able to finish his animation. His spell basically got denied. Sejuani also won't lose her spellshield since she cancelled the spell before the impact.
Zac with spellshield jumps over charging Sejuani
Here Sejuani would just pass through. The spellshield absorbs every effect of sejuanis Q and Zac would simply land where he wanted to land.
Zac and Sejuani (both without spellshields) land at the same spot
Well here it depends on who lands first. If Sejuani reaches her target first, she will get knocked up and damaged. If Zac does, the same will happen with him. However Sejuani has an easy game here since her Qs effect kicks in as soon as she hits something in her path. Zacs E only damages or knocks enemies within his targeted area.
Both of them hit the same spot at exactly the same time
I actually have no idea what will happen then. Both spells are interruptable by CC and will cancel eachother if someone hits first. The chances of 2 of these hitting the same spot at exactly the same time are under 1 in a million but if it happens expect some weird buggy stuff.