Tendril
Tendril has 80 health.
It takes extra damage from fire; it resists frost; poison does nothing to it at all. Blunt, slash and pierce land at full damage, so that is what to bring.
Nothing but a player can damage it, so tames, Dvergr and summoned companions do nothing for this part of the fight.
It has one attack, 55 blunt.
Of everything craftable, 
Thunderblood Crossbow, 
Lightning Strike and 
Staff of Embers put the most damage through what it resists.
Tendril is an immobile growth that 
Kall Fimbulbringer raises around his arena during the fight. It has 80 health, walk and run speeds of 0, is weak to fire, resists frost, and is immune to poison and to all damage dealt by anything that is not a player. Its slam deals 55 blunt with 20 chop and 20 pickaxe attached, so it damages terrain and structures as well as you. It drops nothing, and it clears itself on a timer if left alone.
They cannot move or follow, so they are terrain rather than enemies: note where they are and stop walking through them. 55 blunt if you do. Weak to fire, resistant to frost, immune to poison. Clearing them costs you attention that the King's chains will punish, so unless one is sitting exactly where you need to stand, ignore it and let the timer take it.
With devcommands enabled, this spawns Tendril in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: Tendril · all console commands and IDs