Great Cthulhu

Cthulhu Rising

Cthulhu himself, with the rest of his species, is currently buried in the vast corpse-city of R'lyeh beneath the sea. Cthulhu exists in a sort of living death but while his body slumbers, his mind remains keenly active. His influence can be summoned and under certain circumstances, with enough power, he can be awakened to end the world.

Cthulhu Rising

Cthulhu's final rising will be accompanied by thousands or perhaps millions of starspawn, his kith and kin. He is like unto them, though vastly more powerful. And like the rest of his dread race, Cthulhu can plunge from world to world through space, and so could travel to any planet or be conjured anywhere.

The Devouring

When just awakened, Cthulhu manifests only part of his powers. When Cthulhu first rises, his tentacles and flabby claws can snatch people up and physically eat them.
As he grows, Cthulhu's tentacles and shapes shift greatly in form. He can flick out pseudopods or claws for hundreds of yards to grasp his prey and return them to his maw, devouring them in an instant. Flight is no longer sufficient to escape his presence.
Cthulhu seeks to devour an entire city or small region in order to unleash his full power. Since mountains of protoplasm are by now rising from the sea, he likely faces little challenge in doing so.
At the zenith of his power, Cthulhu's height grows to miles. His head rises above the clouds and his visage can be seen for hundreds or even thousands of miles. His sliterhing, more-than-mountainous magnitude causes earthquakes, sends forth tidal waves, and even attracts storms. Clouds form against his midriff and shoulders and his progress brushes them away.

Telepathy

When Cthulhu is awakened, his mental powers stretch for miles, potentially blanketing a planet. Everyone nearby has visions of Cthulhu's mind and perceives his unthinkable goals, which are largely incomprehensible to mortals. The effects of this telepathy are profound, filling the minds of most nearby humanoids and beasts with consuming terror, hallucinations, and shrieking madness. This telepathy is one reason that terrestrial armies are unable to block or destroy him: most soldiers will just lie screaming on the ground in a fetal position, unable even to draw a weapon.

Great Old One of cataclysms, dreams, and the stars; Chaotic-Evil

Favored Weapon: Dagger
Symbol: Complex rune surrounding an open eye
Temple: Caverns, coastal churches, hidden coves, remote isles
Worshipers: Deep one hybrids, doomsayers, dreamers, insane artists
Servitors: Deep Ones, starspawn

Non-Euclidean Geometry
Cthulhu and especially R'lyeh cover all nearby terrain in a pall of disorienting geometries. Distances change as if in a nightmare. If you seek to flee Cthulhu, you'll find that the path stretches out longer and longer. If his minions pursue you, they'll somehow catch up, like a villain in a slasher movie, as though time and space provide no barriers to their movement. It is not teleportation, however, but the distortion of distances.
The most lethal effect of this geometry is that sharp angles sometimes act strangely as Cthulhu nears full power. They can swallow up individuals, transporting them to other worlds or dimensions. For example, if you are being chased through a forest, you might see someone run behind a tress as he flees, and then never come out the other side. He has somehow "fallen through" the dimensions and is no longer in the material world.