Talk to Grüfeld Bach and agree to help. He will explain that an evil Zamorakian monk, Denath, store an ancient tome of demonology that was used to banish a demon, Delrith. Delrith can only be harmed by Silverlight, which is located in the crypt below the church. Enter the small room in the corner of the church, and climb down the trap door next to the stairs.
Talk to the spirits. Tell them you seek Silverlight. They will tell you you must first pass their trials.
Trial of the Body
To the west, speak to the Spirit of the Body. He will have you pass a combat test. Kill all 10 level 2 skeletons.
Trial of Faith
Head to the north room and speak to the Spirit of Faith. Click onto the chasm behind the spirit and you will walk on it as if it were solid. Take it one step at a time, and listen to the directions that the spirit leads you.
Trial of the Mind
Speak to the Spirit of the Mind, and give him the following answers: [list]
After completing all three trials, talk to the spirits again in the central room. They will dispel the barrier. Take Silverlight.
Speak to Grüfeld again and he will tell you that Denath seeks to summon Delrith beneath the Zamorkian Temple in Varrock. Head straight south from the Saradomin church, past the bank and Aubury's Rune Shop, and look for the Temple of Zamorak in southeast Varrock.
Enter the trap door, and kill the Monks of Zamorak. Continue on through the temple killing all of the monks. Enter the next chamber and a cut-scene will play. It will be too late, Denath will summon Delrith and flee. Kill Delrtih with Silverlight. Lead Delrith a little bit away from Grüfeld, then when he shouts "Prepare to be incinerated", run back to Grüfeld and wait inside his shield, otherwise you will be dealt large amounts of damage.
After killing Delrith, Grüfeld will teleport you back to the Church of Saradomin. Talk to him again.