Islands of Mystery, Chapter 2: Dragon Wisdom

There are several such locations, and each one modifies the hero's stats in its own way (using a script):
A purple vortex on the grass gives +4 to defense;
A purple vortex in the lava lands gives +3 to attack, +1 to morale, but -3 to magic;
A pink vortex in the desert gives -2 to luck and +3 to knowledge.