Once Nevalden Rehn has the crate of marks, you can get some from him at any time. They aren't lore so it's recommended that you grab as many presences as you are currently seeking. Grab 3 to start out with.
In order to rescue the Vah, you need to kill certain areas of a zone. The presences do NOT spawn off just anything in a given zone and some are rarer than others. If you're in the right area, every mob you kill has a chance to spawn a hidden presence upon it's death. If you're flagged to rescue the Vah in that zone, hailing the presence will allow you to spawn the actual Vah you were sent to get. Once you have the real Vah in front of you, talk to it and give it a Mark of the Vah so it can leave.
Don't get discouraged if you can't get one to spawn quickly. And if you're a short race, learn to look up. Presences don't actually have a character model but they are tall. They're just a floating name and are usually well above a short race's head so they're kind of easy to miss. They also don't necessarily spawn the very instant a mob dies. This isn't a big issue if you're pulling all the mobs to the same spot, but if you're just roaming around and killing everything in your path, one might spawn behind you without your notice. The hidden presences only stay up for 5-10 minutes so be sharp about it.
Walkthrough
Find Nevaldan Rehn (Southern Waste of Tarhyl at -1240, 1290, -22 • Vah Camp) .
Repeated Dialog
...
You say, 'So I should talk to the traveler if I want to know more.'
Nevaldan Rehn tells you, 'Exactly. She isn't here now, but she said she'd be back before long. For now, she's told us where to find more of the refugees. There are four more: One in Fire Grottos, one in Everchill, one in Highhold Keep, and one in the First Ruins. Oh, and one other thing, <Player Name>.'
Your journal has been updated. [The Vah]
Journal Text: I've learned from Nevaldan that the traveler is the source of his knowledge of the vah. If I want to press the issue, I can speak with her. Past that, there are more refugees to rescue: In Everchill, Fire Grottos, First Ruins, and Highhold Keep.
Receiving Additional Marks
Note: You can request as many Mark of the Vah as you need from Lyjan at any time.
Find Lyjan Vahl (Southern Waste of Tarhyl at -1280, 1270, -23 • Vah Rehn Camp) .
You say, 'Hail, Lyjan Vahl'
Lyjan nods, flicking an ear at you.
You say, 'I need another mark.'
Lyjan removes another amulet from the crate. He cleans it for a moment, then begins to tell it of the fate of the vah.
You say, '-Let him finish.-'
The shade flees from the mark. The metal lays inert in Lyjan's palm. He closes his fingers around it, claws digging into his palm. 'There's no time for mourning. Vah are still lost out there and they need to be shown the way. No grand cities, no art, no science or culture, tents only, yes. No future, no hope. A beaten people. But people still. Is it right that vah should die alone?' A hint of wavering light shows through the fur between Lyjan's fingers. 'If you and I and our oaths are all that's left, so be it. When we go, it will be as Vah *Shir*. Our people are here and our duty is here and we will serve both until we all pass and the sky is empty again.' The light steadies, pulses once, then fades. Lyjan turns to you and places it in your palm. 'It has been bound to this place. It will do its part as long as it is able.'
You receive Mark of the Vah from Lyjan Vahl.
Complete Saving Hyvek Vahl, Saving Verjya Hakahm, Saving Vyre Sraen, and Saving Sorysia Laythen.
Return to Nevaldan Rehn (Southern Waste of Tarhyl at -1240, 1290, -22 • Vah Camp) .
You say, 'Hail, Nevaldan Rehn'
Nevaldan Rehn says, '<Player Name>!' He claps you on the shoulder. 'You brought them all back! This is incredible. I can't begin to describe how much it means to these people--my people--what you've done.' Seeing your expression, he adds, 'and the traveler, of course,' but you sense that he doesn't mean the last bit. 'Here, I've got something for you.' Nevaldan hands you a mask and a rolled piece of cloth.
You gain 195,000 experience!!
Your faction standing with Vah Rehn Survivors has gotten better!
You receive Mask of the Vah from Nevaldan Rehn.
You receive Rehn Banner from Nevaldan Rehn.
Your journal has been updated. [The Vah]
Journal Text: I rescued the remaining refugees that Nevaldan was aware of. He gave me the banner of the Rehn, such as it is, and a Mask as reward.
You say, 'What are these for?'
Nevaldan Rehn tells you, 'The mask is simple. Marks are only given to outsiders when the vah trust them, and Masks are much the same, though the trust required is far greater. The gift of a Mask allows the outsider to mingle freely among the vah, even with those who are xenophobic or prejudiced. The banner... is complicated.'
You say, 'What's complicated about the banner?'
Nevaldan Rehn tells you, 'Every noble house has a banner. Vahl, Shahar, Kajek, so on. And once, long ago, the house of Rehn was a normal noble house with a normal noble banner.'
You say, 'What happened to the house of Rehn?'
Nevaldan shrugs. 'Some say a rival house defeated them and banished those who would not take their name. Others say the house was plotting rebellion. I don't know for certain.
You say, 'What should I do with the banner?'
Nevaldan Rehn tells you, 'It's not very useful in its current state, but if you can find a pole to put it on, you can wear it like the standard-bearers of old. Lyjan should be able to help you find one of those.'
Your journal has been updated. [The Vah]
Journal Text: I rescued the remaining refugees that Nevaldan was aware of. He gave me the banner of the Rehn, such as it is, and a Mask as reward. I should speak with Lyjan to learn how I can make use of the banner.