According to Kerapac, Strisath was originally one of the Dragonkin race's greatest philosophers. Apparently he was able to infer meaning from chaos and decipher patterns from complexity. This talent was lost when Strisath was driven completely mad by the Curse of Jas.
Strisath initially appears in a cutscene during While Guthix Sleeps where his body is seen to be glowing in a manner just like the player's during the quest after making contact with the stone. He and his fellow Dragonkin Sakirth and Sithaph talk of bringing pain to the mortal races of Gielinor.
He is again spotted during the Ritual of the Mahjarrat quest. Following the rejuvenation ritual, Lucien summons the Stone of Jas. The Dragonkin make their way to the ritual plateau where they proceed to attack the false user of the stone. Strisath is thrown to the ground as he attempts to lift Lucien skyward, before Lucien is stabbed by Sakirth.
After the ritual, the devious MahjarratSliske lured Strisath - who was guarding the Staff of Armadyl at the time - into the Shadow Realm so he could trap him and steal the staff while he was gone. To Sliske's delight, Strisath actually brought the staff with him, and he was easily able to take the powerful weapon for himself. Sliske then trapped Strisath in a cage surrounded by shadows, and used the staff to kill Guthix. Sliske also claimed he used the staff to reveal the Dragonkin's connection to the Stone of Jas, allowing him to locate and retrieve it.
Strisath, angered at the False User.
Sliske later gathered all the gods together within the Empyrean Citadel of Armadyl, before proceeding to mock them all and revealing the Grim Reaper and Strisath, both locked in cages. He used to the threat of killing Death and thereby trapping the Gods' follower's souls in their bodies to control some gods. To others, he threatened to release Strisath. Declaring that the Stone of Jas would be awarded to anyone who killed the most Gods in his upcoming gladiatorial games, he was frustrated with the lack of action upon his announcement. To kick things off, he released Strisath from his cage. Strisath, empowered by the recent use of the Stone, began to attack the player, who had previously touched the stone, which caused the gathered Gods to flee, one by one. After Icthlarin fended off Strisath, the adventurer managed to free Death before the trio teleported to safety.
Following the quest, the Statue of Death remarks that the citadel took some cleaning up as "he really lost his temper" before his rage ran its course. It is unknown what he did after the quest, but before his capture by Kerapac.
Sometime before the events of One of a Kind he was captured again, this time by a fellow Dragonkin named Kerapac. Kerapac took him to his Daemonheim laboratory for study in an attempt to help cure his madness, storing him in a cage-like magical construct. The adventurer present at the Ritual and Sliske's Ascendancy travelled inside the construct to defeat a physical manifestation of Jas' curse, the Echo of Jas. After the Echo was defeated, Kerapac reported that Strisath's rage subsided - albeit temporarily.
^Sliske, "Missing, Presumed Death", RuneScape. "I've been experimenting with the Staff of Armadyl and found it to be an extremely versatile tool. With Strisath imprisoned, I used the Staff to reveal his connection to the Stone, leading me to exactly where you sent it."