Find oracle in Monk Temple was a mission in Jak 3. Jak explored the Great Volcano and discovered a fallen satellite with a petrified precursor monk holding a dark idol. He gains the Dark Invisibility power from the idol, which he uses to enter the Monk Temple
With your new Dark Invisibility power go and jump down into the lava river again, taking the stone platforms for a short tour (don't drop down the lavafall) and jump off again at the end. Go down the slope here only to end up at the very beginning, where you'll have to use the nearby dark idol to go invisible and avoid the sentry and pass through the teleport gate.
Back at the Monk Temple go do the same trick again (since you can't exit through the normal entrance) and head deeper inside, down the stairs. Some widowmakers inhabit this place, at the bottom of the stairs you will find a dark eco Vent which you need to perform a Dark Bomb a little later. In the next room make sure to avoid the center and go around it, the devices around the center of the room electrocute you if you get too close. On the other side go invisible by using the dark idol, then perform a Dark Bomb (or simply use the Wave Concussor) in the middle of the room to wipe them all out.
Dive punch onto the button in the center here to open a nearby door and when you get close to it a lot of spiders will start spilling out the other side. Use whatever weapon you want to plow through them to watch the next room go into lockdown. Here you have to use your invisibility power to avoid the sentries and get to the other side by way of pole swinging. You will have to do this a couple of times, each time de-activating one eye.
After completing this section go and open the next door, where once again you will encounter a swarm of widowmakers. Wipe these all out as well and continue into the larger room up ahead to begin a cutscene, then walk into the light beam and bring another cutscene and receive your first light eco power; Light Regeneration.
Exit through the nearby teleport gate and finally leave the Monk Temple.