The creature in Norse mythology known for guarding the roots of Yggdrasil is Nidhogg. This serpent-like dragon resides beneath the World Tree, gnawing at its roots and symbolizing the destructive forces of nature. Nidhogg's role illustrates the constant struggle between creation and destruction, emphasizing the cyclical nature of existence in Norse belief systems. The presence of this creature connects deeply with themes of life, death, and rebirth that permeate Norse mythology.