There are five total biomes in Valheim that currently have bosses associated with them. Once defeated, these bosses grant the player access to a Forsaken Power that will buff the player's stats in a certain area.
Activating the Powers
In order to get these Forsaken Powers, players will have to have killed each of the Valheim bosses and claimed their trophies. Then the trophy must be hung on the correct hook in the starting spawn area, called the Temple or the Sacrificial Circle. Each boss will have its own trophy stone in the circle. Players can only have one power at a time in their inventory; in order to switch which powers they have access to, players just need to interact with the trophy of the boss associated with the desired power. The F key will activate the power. Now that players know how to get an activate them, it's time to rank the powers.
The Elder
The Elder is a giant, tree-like boss that summons whipping roots and shoots barbed roots from its massive hands. While the boss itself is scary and incredibly dangerous, its power is not as useful as players might have hoped. The Elder's Forsaken Power allows players to chop trees, logs, and stumps down faster for five minutes with a cooldown of 20 minutes. While this buff has a small AOE so other players in the world can benefit if they are standing close by to player who activates it, this still does not make the Elder's Valheim power as useful as the others.
Yagluth
While the future roadmap for Valheim whispers of unknown bosses that will need elemental resistances to defeat, Yagluth's power is currently very situational. This power, like the Elder's is a five minute buff with a 20 minute cooldown, and it gives the player resistance to fire, lighting, and frost. In specific situations, like taking Moder on a second time or heading down to Valheim's Ashlands to fight Surtlings and collect Flametal, this buff is invaluable. However, players will most likely find this buff very helpful in the upcoming expansions to this Early Access game.
Moder
Moder, the icy queen of the Mountain Biome, grants players the power of sailing with speed. This buff grants five minutes of guaranteed tail winds while sailing, helping players that are trying to outrun Valheim's dangerous sea serpents. The cooldown, like all the other buffs is 20 minutes, which makes it a little less useful than it could be. It also unfortunately does not boost the speed of the wind; players caught in light winds will not give more power to the winds in their sails. However, it does make the boring business of sailing a little faster and a little more interesting.
Outside of sailing a Valheim Karve, longship, or a raft, this power does nothing. Players cannot change the direction of the wind while firing arrows, cutting down trees, or any of the other things effected by the wind.
Eikthyr
Eikthyr is the first boss that players will battle against, as this lighting-powered elk god is the boss of the Meadows. When activated, running and jumping cost players 60% less stamina than usual. Like the Elder's power, it also has a very small AOE around the player, allowing them to buff friends that are standing next to them. The power lasts for five minutes and has a cooldown of 20 minutes, but this is plenty to help players get where they need to go and help out in dangerous situations. Eikthyr's Forsaken power does have some limitations, however; players will not see reduced stamina usage for running up steep hills, and it cannot override Encumber status effects.
Thankfully, this power does not have a universal cooldown; a group of four players can keep everyone buffed all the time if they are standing close enough to one another. This power is massively useful, especially in the early game when players will not have access to the food combinations in Valheim that grant massive stamina pools.
Bonemass
Bonemass, the boss of the Swamp biome in Valheim, gives the most useful of all the buffs currently in the game: overall reduction to physical damage. Players take substantially less damage from blunt, slash, and pierce damage, helping to protect them from arrows, clubs, swords, axes, and any other non-elemental attack from enemies. While there are some enemies that do attack with elemental damage, such as oozes and blobs, drakes, and the bosses, most NPCs players come across will be dealing some sort of physical damage instead.
Valheim players get five minutes of almost god-like abilities to survive attacks. While this again has a 20 minute cooldown and no AOE to share the damage reduction with friends, this power is still the best in the game.
Valheim is currently in Early Access for PC.
Source: Wiki