Which Login hook should I use?

I need to write a custom plugin which:
a) as soon as someone logs in, checks their account status (RCP has function for this)
b) IF their account is “expired” and they have a user role of “active”
THEN change their role to “subscriber”.

I think I’m OK with a) and b) but I’m a little unsure about what hook I should be using to fire this stuff.

Any links, recommendations?