Holy shit are those tar growths terrifying. I got back into playing after H&H came out and just beat Moder a few nights ago, and I finally stumbled onto a plains section that had tar pits.
Silly me thought "they look similar to swamp blobs, so they must act/attack like them too", and went in to mash them up with my mace.
Big mistake.
My screen almost went black from the tar shots spewing from the half dozen growths hot on my tail. I try to run, wasting my stamina, but I'm going at a snails pace, taking damage in chunks. I pop a healing potion and strafe just enough to get the slow debuff off me and haul ass into the black forest, barely escaping with my life.
Definitely not even coming close to those tar pits again until I'm fully decked out in black metal armour. Lesson learned.
Source: https://www.reddit.com/r/valheim/comments/rkup28/so_i_went_to_the_plains_after_starting_a_new_seed/