I'm not really sure what you mean with "boss".

A boss in Valheim speak is the enemy you have to beat to get access to the next tier of food, armor and weaponry, so you can conquer the next biome.

No mod can provide that to you. What mods CAN provide is a really tough "mob" that you have to fight like a boss. But they are not a "boss" per se, which, again, is not really possible for a mod.