Gawen

Gawen was the Grand Maester during the reign of Maegor the Cruel. He was the first of three Grand Maesters executed by Maegor.

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

In the books
In The World of Ice & Fire and Fire & Blood, Gawen served as Grand Maester for Kings Aegon I and Aenys prior to serving Maegor. He was the third Grand Maester, being the successor of Grand Maester Lyonce. When King Aenys fell ill at Dragonstone in 41 AC, Gawen had little to no hope in the king's betterment. Gawen protested the coronation of King Maegor, stating that Prince Aegon is the rightful heir to the throne. Gawen was beheaded by Maegor with Blackfyre for his protests.