What is Akito's Geass?
Akito doesn't have a Geass, but he is under a Geass.


  Akito is under the effects of a Geass that he appears to control at
  will, allowing him to savagely kill his enemies in the battlefield. - Codegeass.wikia.com


His eyes also don't have the Geass Sigil inside, but his pupil is encircled red like a Geass has been casted on him.

For the ability to talk to dead, I think it has something to do with him coming back from dead, maybe he defeated death somehow.