Tocasia

Tocasia was an elderly archeologist from Argive on Dominaria to whom both Urza and Mishra were apprenticed as youths. She taught them the basics of artifice and helped them learn of the Thran. She also accompanied them on the fateful journey to Koilos, when they discovered the Mightstone and the Weakstone. It was she that first noted the rising animosity between them after this trip.

She tried to get both brothers to hand the stones to her, a neutral third party, so she could examine them without their arguing, but neither trusted her, each believing she would give the stone to his brother. This distrust and anger reached critical mass one night, when each Brother used his stone to attack the other. Tocasia stepped between the stones, trying to stop the fight. Both brothers lost concentration and an explosion took place, which killed Tocasia. Mishra fled, and the excavation camp fell apart without Tocasia's influence.

Alternate reality
According to the flavor text of Primal Plasma, she was a elementalist in the alternate reality introduced in Planar Chaos.