The Elder is a boss with 2,500 health.
It takes extra damage from fire; poison and spirit do nothing to it at all. Blunt, slash and pierce land at full damage, so that is what to bring.
Its hardest hit is stomp at 60 blunt, out of 2 attacks.
Of everything craftable, 
Staff of Embers, 
Frostfire Crossbow and 
Thunderblood Crossbow put the most damage through what it resists.
It always drops 
The Elder Trophy and 
Swamp Key.
The Elder's altar hides in the Black Forest. Vegvisirs inside Burial Chambers point to it. Burn three 
Ancient Seeds in the offering bowl to summon him; the seeds drop from Greydwarf brutes, shamans and the nests they guard.
The Elder is a ranged fight that punishes the open field. His vine volley hits for 35 pierce per vine, the roots he summons fence you in, and his stomp crushes anyone loitering at his feet. Fire is the answer. He takes double damage from it, so 
fire arrows shot from behind the stone pillars around his altar win the fight at little risk. Keep an eye on the treeline: trolls and greydwarfs wander in, and at night the whole forest joins him.
The woodcutter's power. Activate it before a logging or mining session and watch the yields climb.
He drops the Swamp Key. It unlocks Sunken Crypts, the main source of scrap iron and the reason to fight him at all.
With devcommands enabled, this spawns The Elder in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: gd_king · all console commands and IDs