You need two stag heads to summon Valheim's first boss: the great stag Eikthyr.
When you are dropped off for the first time in the land of Vanheim, you will find yourself surrounded by a pile of stones, and with Huginn the Raven perched on a brilliant red. This is a Vegvisir, and it will mark the location of the biome boss on your map. Since you started out in the meadows, the great deer Eikthyr is the first boss to kill in Valheim.
The location shown on your map is the ritual “sacrificial” stone used to summon Eikthyr, but you will need two heads of his “relatives” to summon him. Since Eikthyr is a deer, this means that you will need to get two deer trophies by killing deer, which are quite common in the meadows. They are easily scared, however, so there are several ways to kill them without having to hunt them all over Vanheim.
The first option involves stealth: after crafting a basic stone ax, sneak in while crouching when you see a deer in the distance (by pressing CTRL). Stealth will consume stamina, but if you stop moving, you'll regenerate it. Just sneak up on the deer and slap it with your ax from behind: tada, easy to kill.
Alternatively, you can try killing them with a bow, although the Crude Bow starter is slow to fire and has terrible range. It's not too difficult to craft, but if you are an enterprising spirit, I recommend crafting the Finewood Bow instead. You can create it early in the game (as detailed here), but it will take a bit of time and some solid dodge skills. If you choose to use the bow - whether raw or thin - you'll always want to use stealth to get closer to the deer you're stalking. Just keep in mind that you are exiting stealth while aiming.
Personally, I have gone for the ax / stealth method, but use the one that best suits your playstyle. Once you've killed yourself a few deer and grabbed a few trophies, you're ready to summon Eikthyr. The point is, Eikthyr will kill you quickly if you attack him without decent gear. He might be Valheim's first boss, but you shouldn't attack him without some new upgrades.
Hunt more deer and wild boars in Meadows to collect deer hides and leather scraps. Take twenty of each, cultivate 40 wood with your ax, and grab 20 stones from the ground. You will need all of these materials to create a workbench, then leather armor. If you want to go the extra mile, also grab some flint from the floor to craft a flint ax once the workbench is up and running. The workbench needs a roof before you can use it, so be sure to use one of the abandoned houses that litter the meadows to settle in.
Once you have a working workbench, at a minimum create the Leather Tunic, Leather Pants, and Round Shield (and Stone Ax if you haven't already). The Leather Helmet and Flint Ax go a long way, but with these items you at least have a chance against Eikthyr. Equip these items, eat some food (to improve your health; create a fire and a cooking grate to grill all the deer meat that is in your inventory), then head to the altar of Eikthyr. To summon it, all you have to do is equip your two deer trophies in a numbered slot, then click on that number while aiming for the altar. The sky will darken, the lighting will strike, and Eiktyhr - Valheim's first boss - will appear.
Use your shield to block Eikthyr's attacks and strike between his blows.
Eikthyr doesn't have any complex moves: he'll lower his head to hit you quickly, and he'll stand up to channel the ongoing storm to create a lightning-infused shock wave. You can block both of these attacks, and as long as you have stamina you will mitigate all damage. Hit Eikthyr with your weapon between his attacks to deal damage, and simply rinse-repeat the block-strike-block rotation until he dies. Note that if you fight Eikthyr near water, he will rise above the land on occasion, forcing you to swim and put your weapons away. Swim away from the deadly deer when this happens and don't forget to press "R" to re-equip your weapon and shield once the tide has subsided.
Once Eikthyr falls before your assault, he will lie on the ground for a few moments before exploding into wood and trophy. The trophy is needed to progress in Valheim and the antlers are used to craft the pickaxe. Vacuum up the bloody goods and return to the Sacrificial Stones (the area where you first arrived in Vanheim; the circle surrounded by four small circles on the map). Once there, equip the Eikthyr trophy the same way you did the deer trophies and click on the number while aiming at the deer sacrificial stone. You'll unlock his Forsaken ability (running and jumping will cost less stamina while it's active) and tasked with hunting down Valheim's next boss, located in the Black Forest.
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