Eikthyr is a boss with 500 health.
Its hardest hit is antler at 20 pierce, out of 3 attacks.
Some of what it throws is elemental, so lightning resistance is worth having.
Of everything craftable, 
Frostfire Crossbow, 
Thunderblood Crossbow and 
Lightning Strike put the most damage through what it resists.
It always drops 
Eikthyr Trophy and 
Hard Antler.
Eikthyr's altar stands in the Meadows, rarely far from spawn. The red-glowing vegvisir at the Sacrificial Stones marks the nearest altar on your map. Lay two 
Deer Trophies on the altar to call him. Like every Forsaken, he can be summoned again whenever you can pay the price.
Eikthyr has three attacks and telegraphs all of them. The antler swipe covers a short arc in front of him, the lightning beam fans out to 20 meters, and the stomp shocks everything within 10 meters of his hooves. Stay at his flank, roll through the swipe, and step in while the beam or stomp recovers. A 
flint spear, a 
crude bow and a 
wood shield are enough. Axes and pickaxes do nothing to him, so bring a real weapon.
Mount his trophy at the Sacrificial Stones and activate it to claim the power. It lasts 5 minutes and affects nearby allies too.
He drops Hard Antlers and his trophy. The antlers craft the 
Antler Pickaxe, your first mining tool, and with it copper, tin and the whole bronze tier open up.
With devcommands enabled, this spawns Eikthyr in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: Eikthyr · all console commands and IDs