- “Greetings! I am the Monitor of Line Installation 1-4. I am 686 Ebulient Prism. My function is to collect and inspect any and all specimens captured within this sector for evidence of immunity to infestation by the Flood. I require your assistance.”— 686 Ebulient Prism to Black-One and Reff 'Talamee
686 Ebullient Prism was a Forerunner Ancilla, and the Monitor of Line Installation 1-4. Prism's functions included collecting and inspecting any specimens captured for evidence of immunity to infestation by the Flood. Assistance from the specimen when inspecting was not required to be voluntary according to protocol.
In 2552, Prism's Gatherers had obtained a member of all of the respective species that had crashed on the moon, which included Reff 'Talamee, a Sangheili; Black-One, a human; an Unggoy and a Mgalekgolo. Prism announced himself to the subjects and explained that they were to be dissected and catalogued for study to determine if any of them possessed immunity to the Flood. The Mgalekgolo, being a hive organism, was immune to the Flood, but lacked mass and cognitive capability to satisfy, therefore, he liquefied the Mgalekgolo. Prism was about to turn attention to Black-One, when Reff insisted that the Spartan did not believe in the Great Journey. Prism instead dissected the Unggoy.
But before Prism could continue, he was alerted to the intrusion of the rest of Team Black and Covenant, who had formed a temporary alliance to rescue their comrades. Prism remotely hacked into Victor-101's helmet and tricked him into seeing the enemy everywhere. Also, Prism sent out a false distress signal to Black-Two, who rushed from the remains of Long Time Coming to the Line Installation, before tricking Victor into shooting her. He also destroyed Iona's secondary interface, intent on destroying her completely. Prism chased Iona through Victor's Mjolnir armor systems, using virtual weapons against her. However, Prism became distracted with a visual depiction of Team Black's training days, composed of data from Victor's partially altered logs.
Back in the physical world, Black-One insisted on Prism putting her out of her misery, which the Monitor was gladly willing to accomplish. However, Black-One used her helmet to deflect a laser right at her spare ammo pack sitting on a Gatherer, causing it to explode and crash into her cell. She then used the Gatherer's weapon to free Reff. Prism attempted to stop them, only for One to fire at him, forcing Prism to withdraw from Victor's armor. Summoning Sentinels to his aid, Prism chased the Spartan and Sangheili throughout the Installation. Black-One was able to destroy the Sentinels, only for Prism to shoot her down. Reff, taking her weapon, took out Prism's eye, declaring him to be a false Oracle. They then evaded the Monitor in the Installation's control center. Prism, assembling swarms of Sentinels and Gatherers, soon found them and demanded that they step away from the controls. He vaporized Reff from the legs up, as he was proclaiming about how he was the "Chosen One." Team Black reunited and were able to push Prism back, before using the Installation's main weapon to vaporize the Monitor.
- 686 Ebullient Prism shares many similarities with 343 Guilty Spark, the Monitor of Installation 04. The '"686" in Ebullient Prism is a reference to the "343" in Guilty Spark as it is double the number. Like 343 Guilty Spark, Prism is unethical in a sense as he has no regard for a being's inspection consent. In addition, after being severely damaged by Reff using a Sentinel Beam, Prism said exactly the same line ("My eye! Do you mean to blind me?") as 343 Guilty Spark in a similar situation.
- Halo: Blood Line (First appearance)