Cylaces or Glak (Old Armenian: Գղակ, romanized: Głak, misspelled Դղակ, Dłak in the manuscripts of Faustus of Byzantium's history) was a 4th-century Armenian eunuch who occupied the office of hayr-mardpet (grand chamberlain).
He initially defected to the Sasanian side during Shapur II's invasion of Armenia, but later rejoined the Armenians in 368 and fought Shapur II's forces.
[1] In 370, he sent messengers to Shapur II, promising him to betray the Armenian king Pap (r. 370–374).
However, this ploy was discovered by Pap, who had Cylaces assassinated.
[2][3] In Faustus of Byzantium's History of Armenia, Cylaces is split into two characters, both of whom are executed on Pap's orders: an unnamed hayr-mardpet who is executed for insulting Pap's mother Parandzem, and another one named Glak who is executed for conspiring with the Persians.