Kathulos was a seemingly sapient planet that existed countless light years from Earth, under the service of Shuma-Gorath the creature launched an attack on Doctor Strange via sending a band of winged demons to assault the sorcerer supreme as he was visting Stonehenge.
Although Doctor Strange easily defeated this lesser demons he soon found himself teleported directly to Kathulos, who immediatedly tried to overwhelm Strange with its mastery of its own biosphere, covering him in vines and mesmerizing him with its psychci powers, attempting to feed on his astral-energy, but would expose its weakness in the process as Strange used the Crimson Bolts of Cyttorak to attack Kathulos' root system, causing the monster to wither and apparently die : Strange was left stranded on the now lifeless planetoid but would eventually find a means to escape.
Kathulos is a typical demon and is thus ruled primarily by malicious intent and enjoys the suffering of others, it is also murderous in this regard and seeks to please Shuma-Gorath by killing the demon-lord's enemies : Kathulos is arrogant, however, greatly under-estimating its enemy and thus costing it dearly as its reckless zeal to destroy came at the cost of exposing its own weak points.
Kathulos is a planetoid drifting far, far from Earth but can shift parts of itself to form structures (much like other "living planets") - most notably Kathulos can appear as a floating humanoid head within a venus-fly trap of gigantic size, this appears to be Kathulos' sole weak point as well, since if uprooted it will cause the entire creature to wither and presumably die.
Kathulos is a powerful mystical entity that has displayed the ability to summon demons, teleport Strange across many lightyears and call on aid from Shuma-Gorath to increase its abilities - when a victim is on Kathulos' surface the entity can manipulate its surroundings and attack victims on the astral-level, consuming their astral-forms if they are weakened enough.
- Kathulos and the Old Ones are all based on the works of H.P. Lovecraft, altered in order to fit into Marvel's extended multiverse.
- other "living planets" in Marvel include (but are by no means limited to) Ego, Id the Selfish Moon, Living Monolith, the Topographical Man, Super-Ego and Marduk.