My friend is pretty sure Cho'Gath's Vorpal Spikes does proc Rylai's Crystal Scepter's passive, but we both can't test at the moment. I thought it didn't proc on toggle skills, but he swears it does. The people in the threads I mentioned seem to classify Vorpal Spikes as an AoE spell rather than an on-hit toggle skill... Anyone have some insight on this?
I'm thinking it does add the slow from Rylai's. I've been looking on the wiki's and I found this:
The following will not trigger a slow:
- Champion abilities that do not trigger spell vamp
- On-hit damage damage dealt by items
- Summoner Spells, such as Ignite and Smite.
- Damage dealt by the auto-attacks of Clones and Pets/Minions.
Then on this page http://leagueoflegends.wikia.com/wiki/On-hit_effect#Items I found under:
Abilities — do apply spell vamp:
Area Of Effect
The following effects deal area of effect damage, but are not classified as splash damage: Cho'Gath's Vorpal Spikes