Alduin basically reincarnates each time he's killed (even when he's killed by dovahkiin, making him totally unbeatable), he can feed on people's souls (don't let him reach the Hakugyoukourou), and he can ressurects other dragons, as well as using the Voice (basically giving him a whole crapload of powers, including a thunderstorm, a mist, and some other things).
Alduin basically reincarnates each time he's killed (even when he's killed by dovahkiin, making him totally unbeatable), he can feed on people's souls (don't let him reach the Hakugyoukourou), and he can ressurects other dragons, as well as using the Voice (basically giving him a whole crapload of powers, including a thunderstorm, a mist, and some other things).
He'll as hacked as anyone else in Gensokyo.
Nope, it's just Alduin is situated out of the time, so he can spawn just now. But if you seal his soul in Sovngarde, he can't go out of Sovngarde.