who is the aka-tusk i read about him in the aludagga and read somewhere else alessia made covenant with him on her death bed. but who is he?

Aka-Tusk is the Nordic perspective of the original Aka that created the world, in contrast to Alduin who split off and consumes it (thereby becoming the Aka of the next kalpa, as may be theorized from Shor son of Shor), Akatosh (the Cyrodiilic perspective of Aka/Alessian religious creation, who may or may not have been influenced by the Dragon Break associated with the Marukhati Selective), and Auri-El (the Altmer variant, who may or may not have been influenced by the Dragon Break associated with the High King of Alinor).

If that sounds complicated, it's because it really is. Summary, peel back the layers, add some Nordic perspective, and you get Aka-Tusk.

He's also referred to as Borhamu by Dragons, and as Akatosh for TES players that want a really short version.