Hel's Bramble is a thick black, vine-like thorn plant that is presumably native to Helheim and is incredibly tough to break. It can be found growing in bushes several times throughout Helheim, usually blocking a path you need to get through or hiding treasure. The only safe way through it is to light it on fire with the Blades of Chaos.
It's implied by Mimir that Hel's Bramble is almost completely indestructible with its only known weakness being magical fire. It may also be poisonous to any mortal or magical being who makes physical contact with it.