Saya raises the spear to point up, but stay ready if they have to fight. 

"And how do you plan to leave?  This is the domain of Erebos, god of the dead.  Or do wish to become one of the returned? " He responds.

The god Phenax, once a mortal who died and passed into the Underworld, was the first to discover a way to escape, forever known as the Path of Phenax. As part of his escape, he swam across the Tartyx, whose waters stripped away his identity. Without that vital part of his being, Phenax couldn’t be detected by Athreos, and Erebos couldn’t snare him with his great whip.

Since Phenax’s escape, other souls have repeated his dangerous journey. When mortal souls travel the Path of Phenax, the Tartyx washes away their identities, symbolized by their faces, which become nothing more than blank flesh. Souls that successfully emerge on the mortal side of the Tartyx River become Returned, with no knowledge of their former name or past life. 

Or you can wait a moment.  Erebos himself has a proposal for you.

"A lot of Gods seem to be interested in our actions." says Areus looking at the rest of the group "I do not have a problem in waiting to hear Erebos's proposal." The Akroan pauses a moment before continuing "However, if becoming a Returned is the price to save Theros, it is a price that I am willing to pay."

Saya approaches Areus. “I am hoping that we do not need to become a Returned, but I am with you. Whatever it takes to accomplish our mission. Though, I fear if we do become one of them, we would not be able to continue without our memories.” Then she whispers to the Akroan’s ears. “Didn’t Erebos create the masks?” Saya returns to attention, ready for a fight, if necessary.

The multiheaded dog grinned and chuckled.  "Indeed centaur, he did craft the masks in an age past.  You have saved Theros by stopping Calisto.  But to bring the masks back to that plane would put Theros in danger again.  And if you were Returned you would lose who you are and would not recall why you had the masks. "

"Here is his proposal.  Erebos will restore you to life and in exchange you must leave both masks in the underworld.  They will be safest out of the hands of mortals. You must swear an oath to never praise Heliod and you may never look upon the rising sun again."

"We technically fulfilled our oath to Kruphix as Calisto was not killed by our hands. Eftalia delivered the killing blow." says Areus as he hears Erebos's proposal. "Erebos, being the creator of the masks, has right to ask us to leave them here, however, if I recall the myth correctly, in the past the masks were stolen from the Underworld."
The Akroan takes a step towards Kuronos "The safest way to protect the masks is to not put them in the most obvious place. If Erebos allows us to return to Theros with our memories intact, I swear on my honor that I will guard the fragment in my possession as long as I am capable of."

Soren shakes his head "My friend is partially correct. The masks are not safe here. They are not safe anywhere. The only way for Theros to be truly safe is for the masks to be destroyed. They were created from grief and they bring turmoil and torment to all those around." 

No soldier of Akros, you are mistaken.  At the height of Archonia’s power, that empire of virtue which held Theros in its merciless grip, the tyrant Agnomakhos set out to find the masks. Legends diverge whether the tyrant never found the masks, or if heroic Kynaios and Tiro returned the masks to the underworld after they defeated the Archons. Either way, Erebos feared the masks falling into the wrong hands. He broke the Elpis mask into five pieces and scattered them across Theros. Some pieces he hid where only gods may tread, others he gifted to Theros’ greatest heroes, and yet others he gave to dread monsters to guard. The Ainoe mask, which resisted all efforts to destroy it, he gave to the goddess Ephara to keep safe.

But you found the Ainoe mask and gave it to Calisto and then were bidden by the gods to make your error right and find the other fragments to keep Calisto from gathering them as well.  You have succeeded in your quest.  Erebos now bids you leave the masks, his masks, with him in the underworld and he will protect them.  They are not meant for you and they are not meant for the people of Meletis.  You are heroes of this age, but some of those in Meletis may seek to use the masks as tools to their own ends.  The masks cannot remain in the mortal world.

If you will not agree to his terms Erebos has one final offer. You may enter the world of the living if one of you remains in the underworld. This sacrifice must be voluntary, one life is a small price to pay to save Meletis. There is no way to return to life without Erebos’ blessing. Your only other option is remaining in the underworld, or seeking out one of the rumored secret exits, to travel back to the world of the living, without Erebos’ blessing, as a returned.

Choose now how you wish to proceed.

Saya steps forward. “I Saya, of the Pheres Renegade band, volunteer to stay, while my friends get to go home, so that they are able to finish the mission.”

"I say we leave the masks with Erebros - after all he made them!" Tisto says to the party before turning to Kunoros "if we leave the masks here in the underworld how do we know that they will be safe"

Areus walks next to Saya, he looks at her and at the rest of the group "No, Saya, if someone has to remain to allow the rest of us to return to life, I volunteer." he pauses for a moment "after all Calisto took the mask because I was not strong enough." smiling he continues "Together we stopped Calisto and Eftalia from taking the masks. For the first time in many years, the masks have been reunited with their creator. I understand now, we have done our part, Erebos can guard the masks from now on."
The Akroan takes a few step towards Kuronos "However, if Erebos requires a guardian for them, this warrior of Akros, will gladly remain."

Soren shakes his head "No. If the masks cannot be destroyed, then they will be safest here with Erebos. I also have one additional favour to ask. A long time ago, a man was struck down by my hand. In exchange for my life, I would have him returned to his wife and son. It is time I finally repent for my sin." 

How will you know they are safe?  Their safety is not your concern any longer.  You were to stop Calisto and you accomplished this.  Your lives are finite, and you will have long crossed the river Tartyx, ferried by Athreos the River Guide, to your new home in the Underworld before another threat to the masks appears.  Take your rest and the rewards of your fulfilled quest.  

The centaur has offered her life in exchange for your freedom from this plane.  Erebos is prepared to accept this if you leave the masks here.

Kunoros turns his attention to Soren.

Erebos is not offering the lives of those who have already crossed the river.  You must bear the responsibility of your actions. You have repented but an exchange cannot be made. 

"Saya, there is no need for you to offer your life for ours, we have accomplished our quest, if we leave the masks Erebos will free us from this plane." says Areus at Saya.
The Akroan turns to Kunoros "That was Erebos's first proposal if I am right." The hoplite continues "and if I am not, I ask to not be returned to Theros, I will remain here with Saya."
Areus turns towards Saya and with a smile says "Friends should stay together no matter what happens."

"Well... If the masks are no longer needed, then we can just leave them with Erebos. Right?" Saya asks the group. "However, if that is not the case." She looks are Areus. "Are you sure you want to stay in this place? I don't mind the company. In fact..." The Centaur looks around. "We could flip it, and make a killing in this buyers market, you know." She rests a hand on the Akroan's shoulder, and smiles back.

Tisto looks over and says "you know sometimes they say three's a crowd but i think not in this case! I'll stay as well. Think of the fun we can have down here?" 

"It's decided then, if we can go all back to Theros by leaving the masks in Erebos's care, we will do that. If we can not, we three will remain here." replies Areus at Saya and Tisto.

Soren growls "Screw that. There is nothing more for me back there than there is here. If you three stay, so will I."

You are correct Areus of Akros.  Erebos will restore you to life and in exchange you must leave both masks in the underworld and you must swear an oath to never praise Heliod or ever look upon the rising sun again.  In this way all of you will be free of this plane.

or

One of you may stay in exchange for the others lives. You will leave the masks here and Erebos will restore the others lives with no other requirements.  The one who stays can pay the river guide, cross the Tartyx, and enter their final resting place.

or

You may remain.  Escape from the underworld is near impossible but you know that the Returned have done so. You know you would lose all of who you are in the process.  The returned are hollow shells inhabited by grim and purposeless spirits. These Returned are separated from their memories, which become wandering eidolons. Their second life is theirs to do with as they please, but it’s typically a confused, cursed life, plagued by frustration, bitterness, loneliness, and melancholy. Such leads many Returned along dark paths.

Of course if you choose to remain in the underworld you can pay the coin to the river guide and cross the river Tartyx to your final resting places.

If you cannot or will not pay the coin to cross the river then you wander aimlessly within becoming vacant, misery-worn souls who have endured decades of existence in the Underworld. Your souls eventually permanently turn to stone after enduring numerous deaths in the Underworld. Your corpses linger, petrified until they crumble to dust.

"I guess the question of the century here is. Do we still need the masks?" Saya asks everyone.