Cthulu's Grasp creates green tentacles on nearly every one of your opponent's pedestals. When they step on a pedestal with live tentacles, they take damage every couple of seconds. This can be used to force or prevent movement, as well as to cause the last few points of damage to defeat your opponent.
Cthulhu's Grasp tentacles can be dispelled instantly, but beware of distance. You might get 3 or 4 of them at once in a small map, but in the big maps the area might not dispel pedestals farther away, leaving one or two still hazardous to your health. Also 1.5 fully charged shots or 3 uncharged shots can destroy the tentacles leaving the pedestal still glowing green but safe to use.
These can be hard to hit at lower elevations. Because the pedestal tentacles always activate on every pedestal you're not currently standing on, if you see your rival drawing the rune for the tentacles you can mitigate this high pedestal targeting problem by heading to the highest pedestal available allowing you a high vantage point from which to clear pedestals. Or, if you hear Cthulhu's roar when your rival pulls out the artifact, then start casting dispelling censer right away and grab at least 1 growing green orb. Decide to finish the censer and to throw as soon as you see the Grasp has indeed been cast.