A'lars was the second son of Daina and Kronos, who are the two of the first generation of the Eternals. He was born in Titanos a long time ago in Asia. Some estimations of his age report him to be about 25,000-years-old. Him together with his brother Zuras lived in a civil war wherein their father was on the winning side. They have studied about the informations about the cosmic energy forces and later on became a scientist himself alongside with his father.
After the death of their father, he and his brother Zuras held an assembly which chose Zuras. A'lars chose to leave instead of having the same vendetta that caused the civil war between Chronos and Uranos. He went to Titan and formed an underground assembly himself.
Arriving at Titan, he discovered the sole survivor of the civil war who was namd Sui-San, a Uranian. Sui-San bore children to Mentor, namely Thanos and Eros (Starfox). She later died after Thanos' attack on Titan, where he brutally murdered her. Mentor would often visit her grave site to think and spend time with her.
When Arthur Douglas was killed by Thanos, Mentor and his father Kronos seized his consciousness and transferred it into a newly created body. This new being became known as Drax the Destroyer and he was created for the sole purpose of destroying Thanos. Later on, he met Rick Jones and Captain Marvel to help him in Titan alongside with his son Eros. He was then imprisoned by Thanos but freed by Mar-Vell. During Mar-Vell's last hours from cancer, he was the one who have played important role to taking care of him before he died.
He later told Silver Surfer about the story of Thanos.
Silver Surfer (1998)
In the Silver Surfer animated series, Mentor was portrayed as the brother of Thanos. With his companion Drax, he long sought to find the Universal Library of the Watchers. With the Surfer's help, he was able to find it, only to learn that the only way to obtain the information was to become a life form known as a Viral, which the Watchers had all transformed into and become a single consciousness. Since there was no escape, Mentor became a Viral.
Mentor was voiced by Cedric Smith, who also provided the voice for Professor X on the X-Men animated series.