Depending on the enemy team, choose whether to a Force of Nature or a Thornmail. At the end of your build sell your Doran's Shield to make room for your Rabadon's Deathcap. If your teammates will not benefit from the spell vamp and ability power aura from Will of The Ancients, get a Hextech Gunblade instead. Getting a Hextech Revolver is a very good to get early gane because it helps you farm easier and longer without wasting your health to use the spells, because you get most of it back with the spell vamp, and the health regenerstion from your Doran's Shield will regenerate the rest.

You should ALWAYS take Ignite because it is amazing when paired with your ultimate, and it will also help you 1v1 champions with alot of lifesteal because of the health regeneration reeuction on Ignitee. I always choose to take Flash for my second spell, but Ghost is also a viable choice for chasing down champions, espicially if you need to finish them off with your ultimate on them. Ghost is also a good way to escape but lets be real, Once you hit level 11, you don't need to run because Morde is damn near unkillable then.