Eikthyr is the first boss in Valheim, and depending on your skill levels, you may not have an easy time defeating him. The creature appears as an abnormally large stag with iron chains wrapped around its electrified red antlers. It's much larger than the average Deer and will attack you once summoned.
Elkthyr has a large health pool of 500 HP and does considerable damage with each attack. To make things more terrifying, it can summon lightning to use in combat (which is not something you want to stand in front of when activated).
All in all, it's not too tough to take down Elkthyr, but you will need to be prepared. There are a few things you must do to ensure your first encounter with him ends on a successful note and not the other way around.
Note: This article has been written based on my personal experience in Valheim. Opinions may vary from player to player.
Everything you need to know about Elkthyr in Valheim

Once you've set up your first base in Valheim and have a basic understanding of things, your first major task will be to defeat Elkthyr. This is important because without defeating him, you won't be able to progress and unlock better gear. While the Meadows are a haven, you cannot stick to them forever. With that in mind, let's get to the first part of this guide.
How to find Elkthyr's Altar in Valheim

To summon Elkthyr, first find his Altar. This is easier said than done because it can spawn at a random location in your game. However, for the most part, Elkthyr's Altar should be next to your spawn point when you start the game. If not, you may need to explore a bit (which is never a bad idea).

Once you find it, it will be permanently marked on your mini-map for future reference. As shown in the image above, Elkthyr's Altar spawned close to my own. I was able to find it on the first day of exploration. If you cannot find it on the first day, don't worry. I would suggest you focus on gathering resources such as Wood, Stone, and Flint, as you will need them for the next part of the guide.
How to summon Eikthyr in Valheim

Once you've gained a few levels of combat using the weapon of your choice, it's time to put your skills to the test. To summon Eikthyr in Valheim, hunt Deer to obtain two Deer Trophy. As simple as this sounds, it will be a difficult task initially. To make hunting Deer easier, consider using a Crude Bow paired with Wood Arrows.
You'll need a Level 1 Workbench, Wood, and Leather Scraps. The former can be obtained by cutting down trees, while the latter will require you to kill Boars. If you're lucky, you could find some Leather Scraps in chests while exploring.

After making the aforementioned weapon, you can now hunt Deer. They can sense you coming if you don't sneak up on them. Even if you do, they can still get startled and run depending on the direction of the wind. Make sure the wind is not blowing in the Deer's direction and you are in front of it. I would recommend leveling up your Bows skill to ensure a one-shot kill.

If you're lucky, when you kill a Deer, they will drop a Deer Trophy alongside Deer Hide and Deer Meat. You can use the Deer Hide to upgrade your gear, and the Deer Meat can be cooked to give you an HP boost.
Once everything is in order, consume some food. I would recommend Cooked Beer Meat and Cooked Boar Meat for extra HP, and Honey for extra Stamina. Grab two Deer Trophy, and head to the Eikthyr's Altar.

Once there, interact with the sacrificial/ritualistic stone and place two trophies atop. Ensure the Deer Trophy has been slotted into one of your usable item slots. Otherwise, the message shown in the image above will pop off on your screen. If you've done everything correctly, congratulations, you've successfully summoned Eikthyr!
How to defeat Eikthyr in Valheim

There is no fixed way to defeat Eikthyr. However, some methods are easier than others. If you've focused on Bows, using Wood Arrows, Fire Arrows, or Flinthead Arrows are all viable options. Keeping a safe distance from the target is never a bad choice, but do keep in mind that Eikthyr moves around a lot. Shooting him from afar will not be easy.
My recommendation would be to use a combination of Wood Shield or Wood Tower Shield, paired with a Flint Spear. Train Spears and Blocking skills to at least Level 15 each, and you should be able to fight Eikthyr solo. Of course, it is better to bring along a few friends as they will be able to help you bring the beast, but that's up to you.

If you use my method to defeat Eikthyr, you'll be happy to know your shield will be able to block lightning attacks as well. Just remember to train Blocking to ensure combat goes smoothly. Upgrade your shield to block more damage in combat. Also, dodge the lightning attack, but you'll need to be quick on your feet.

With a bit of luck, timing, and brute force, you will be able to defeat Eikthyr on the first attempt. You can also sleep before summoning Eikthyr, as that will grant you a bonus to HP regeneration, which will come in handy. For your bravery, you will get a few rewards, which we will also address in this guide.
Rewards for defeating Eikthyr in Valheim

After you defeat Eikthyr, you will receive two items: Eikthyr Trophy and Hard Antler. Let us start with the latter. Once you get Hard Antlers, you can use them in conjunction with Wood to create an Antler Pickaxe. This can be used to break Stone and mine Copper. This will allow you to craft more powerful items and expand your settlement in Valheim.

Coming to the Eikthyr Trophy, once you mount it on the respective stone in your spawn circle, you will gain access to Forsaken Power. When used, you and nearby allies will use less Stamina (60%) when running and jumping for the next five minutes. However, keep in mind that once used, there is a cooldown time involved before you can use it again.
That's about everything you need to know about defeating Eikthyr in Valheim. Since it's the first boss, bringing him down is easy. However, as you progress, you'll encounter bosses that are more difficult to beat. Nevertheless, savor this first victory for it shall not be your last if you plan for each battle (and maybe bring along a few friends).
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