Need an ability for my Nexus for the playable Burning Legion, ideas and help.

When the burning nexus reaches Tier 3, I want it to have an ability.

which is very close range to the portal and will either.

a.) pull them into the portal and kill them...

b.) pull them into the portal, permanently polymorph them into a demon and switch to that players control...

Which idea do you like better? and any ideas of how I could create this ability and/or what triggers I would need?

Might not exactly be what you had in mind, but I think it would be really cool if your Nexus took control of the unit for a short time... And then killed them. Kind of a "best of both worlds".

And to do that I'd make the spell trigger-driven and have it do two things. 1, cast Illusion on the target (and set the Illusion to do full damage and not take extra damage). 2, kill the target (without death animation).
That let's you "control the unit" as long as the Illusion lasts. ;)