Mephasm

Mephasm is a pit fiend, polymorphed into the form of a half-elf, who is in the service of Ammon Jerro.

"'Half-elven at first glance, this handsome man's calm, nonchalant demeanor and perfect posture are more celestial than sylvan. Whatever angelic exterior this man wears, it starkly contrasts his reptilian eyes which send chills down your spine when they meet your gaze.'"

Official Campaign
Mephasm is a retainer of the Still Lord, Prince Levistus, ruler of Stygia. Ammon Jerro had made a pact with Levistus, and Mephasm become one of the warlock's many servants. Mephasm, along with Zaxis, accompanied Jerro to the githyanki hideout in some old Illefarn ruins north of Neverwinter. He had set up a barrier field as a back-up plan in case Zaxis failed to breach Zeeaire's portal chamber. During the raid, the PC and party arrived searching for Shandra Jerro, who was captive by Zeeaire. Mephasm was being harassed by a few succubi as the player entered the inner complex. After the succubi were killed, Mephasm, claiming to be bound there by the githyanki, instructed the player to "release" him by stating his "truename: Mephasm" so the barrier will fall. As he departed, the barrier coalesced into an "interdimensional sphere," which he gave the player as a parting gift, noting that they "will meet again."

The PC encounters Mephasm again at Jerro's Haven, where he admits he wasn't completely honest at the githyanki caves and was in the warlock's employ. He tells them that in order to reach Jerro's sanctum, he'll need to get the other fiends to activate a portal.

After the confrontation with Ammon, Shandra begins to break the fiends from their bonds in order to stop her grandfather from killing her comrades. Ammon attacks and kills Shandra in front of Mephasm. As Ammon was stunned by her calling him grandfather, Mephasm told him the truth of his actions, stating that "Blood finds a way, Ammon. Always." As he began to depart back to Baator, he parted stating "when one carries laws too far, they will take you where I go now."

In Act III, it is possible for the PC and Ammon Jerro to summon Mephasm at Crossroad Keep. At the keep, Mephasm will offer to grant the player enchantments, in exchange for his/her "most prized weapon."

Personality
Unlike the other fiends Jerro had in his Haven, Mephasm remained relatively cordial and polite, even to Ammon. He seemed to be the most helpful to the party, even warning of possible danger to Shandra. As the fiends were freed by Shandra's actions, he commented to her that what she did was brave and foolish. When Ammon killed her in his rage, instead of taunting him, he simply chided him for the deed, seemingly disappointed by his rash assault. Whether this behavior was manipulation for an ulterior motive, or an unusual sense of kindness, is unknown.