Later, this figure's potion helped a man disguise himself as Gorlois and have a son by Igraine; that man was (*) Uther, and this man was imprisoned in a tree by one of his students.  
This figure was originally portrayed as a prophetic wild man living in the Caledonian forest in the sixth century, and Vortigen's soothsayers planned to sacrifice this figure and sprinkle his blood on the ground.  Instead, this figure detected an underground pool that contained two different colored dragons.  Later, this figure's potion helped a man disguise himself as Gorlois and have a son by Igraine; that man was (*) Uther, and this man was imprisoned in a tree by one of his students.  For ten points, name this legendary wizard who enchanted Excalibur to ensure that none but Arthur could draw it from the stone.
This figure was originally portrayed as a prophetic wild man living in the Caledonian forest in the sixth century, and Vortigen's soothsayers planned to sacrifice this figure and sprinkle his blood on the ground.  
{Merlin}