It's the mainboss of chapter 9 in FE4.
I found this fight almost unfair because of two things:
continue - He's able to attack twice. The main problem is that his Gungnir kills everyone with two hits, some units will be oneshotted.
prayer - That's even worse. If he's on low health, it's almost impossible to hit him. At least in my run no one could. His 5 authority stars make it even worse. Even balmung didn't have a good hitrate. The maximum was 76% with balmung only thanks to Nanna's charisma skill.
It took me like 100 attempts to kill him I had to finish him off before he reached prayer mode. Thanks to Gae Bolg it was possible.
Seriously it was the hardest and most annoying bossfight to me of all FE games I've played yet.
It's too luckbased for me mainly because of his continue.