Myros

Myros was Grand Maester during the reign of Maegor the Cruel. He was the second of three Grand Maesters executed by Maegor.

Biography
Myros was beheaded by King Maegor for treason, having disagreed with the king.

In the books
In Fire & Blood, Myros spoke against King Maegor's marriage to Tyanna of the Tower, calling Ceryse Hightower his true wife. Maegor silently listened, then killed the maester where he stood with Blackfyre.