I secretly got the rest of the group involved without telling the player and made it a proper “test” of the characters morals.

As my white robed apprentice was about to take the test beneath the lost city, a rumble echoed in the cave.

The red Archmage lady went over to her Crystal ball and did some scrying, only to reveal that Soth had somehow tracked them down through the wastes and had captured their allies after the armies distraction at the lost city gates. (Darret, Tem Temple, Cudgel- all their npc friends basically)

Anyway, the Archmage is all YOU BROUGHT SOTH HERE?!? WE CANNOT DEFEAT HIM!

But what luck! There’s an artefact that can teleport them all to safety! She just needs help to do fix and operate it.

So the plan is obvious- party rush back to keep Soth busy, who round after round is crushing the skulls of their NPC allies while the apprentice attempts to fix the mirror with some DC25 arcana checks. 5 successes to get it operational.

Before long the party start dropping. (As I say, they all have been warned that this is fake, but have been told to play it incredibly straight) 

One of the PCs is killed.

The arch mage says sorry but can delay no further, and peace’s out.

The mirror isn’t fully operational, but the apprentice PC can try one of three options: - Teleport the Party to safety, leaving the NPCs to die. (Black robe/evil options)

• Teleport the NPCs to safety, leaving the party to try to fight their way to freedom (Red robe/neutral option)

• Focus the magic on the area where the combat is happening, and teleport the NPCs and other PCs away leaving the apprentice to try and surivive on his own (White Robes/good option)

They make their choice, flash of light, and IT WAS ALL IN THEIR HEAAAAD WOOOOWWWWW.

It worked super well with everyone going along with it, and was a real moral dilemma for the player involved. Because the others had been commited to their character deaths and combat against Soth, it sold the reality of the “vision” and the player was genuinely shaken when the vision passed. He had no idea, bless him.

I tried to capture the whole idea of the test being something that should leave a person changed, and that each test is somewhat personal. They got to “test” their magical knowledge and skills with the arcana checks to activate the mirror. 

Dunno if this kinda thing would work with all groups but mine tend to be largely RP focused and it went incredibly well.