The Slaying of the Writhing Mass

The Doctor and Charley become trapped in a temporal traffic jam, caused by time tourists come to witness the pivotal event in the history of the planet Ileiko: the Slaying of the Writhing Mass, the alien entity that enslaved the Ileikans in their infancy.

The legendary hero Salan killed the Mass with a single stone, setting the Ileikans free. But what was the Mass, exactly? And what if the legend was wrong?