You need to talk to an NPC who gives you a potion or maybe it's a spell. Anyway, find that NPC, then you can find what you need after talking to them.

If you haven't done so already, you need to complete Ripple Effect first, which is given by Conjurer Grahla at the entry to Glenumbra Moors -> http://esomap.uesp.net/?centeron=Glenumbra+Moors&world=Glenumbra
A Step Back in Time should be given by someone in the Crypt (also marked on that map) when Ripple Effect is complete.

You gotta listen to the clue the storyline gave you involving "fire"