What was the usage of the Malleus Mundi? What could it do and why was it so coveted?
It had most (all?) of the most powerful spells in the world recorded in it. It could bind gods, destroy the world or remake it in the user's preference. It has been recovered and lost so many times that some people think it may be 'sentient' (of a sort), and causing the chaos surrounding it is a way of continuing its existence.
How powerful was/is Rularuu?
Basically a god - he can create worlds, and destroy them.
Who are/were the confirmed gods & goddesses in City of Heroes? Did we have touches of all sorts of mythologies or were some not shown and so forth?
The devs pulled deities from numerous faiths, and warped some of them to their own designs.