Amulet [The Infernal Devices Betweenian One-Shot]

Death is both a blessing and a curse. Death takes, takes, and never gives-until you are the one to die.

Because Death might well lead you to another world, a Purgatory the likes of which you've never seen. And, most importantly, you can rejoin the ones your heart longs for.

This is precisely what happens to Theresa Gray, well-known, powerful warlock-who happens to have two great loves, not one. The only problem? One of them is invisible to her, and will be until they reach the True Afterlife.
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Will was ecstatic when Jem turned out to be his Guardian, and vice versa when the latter discovered that his parabatai was his charge. What could be more fitting that their twin souls guiding each other to the True Afterlife? This way, they'd remained intertwined in the same way that they did in life.

They didn't see the problem until they found Tessa again.

Until Jem could not hold her again.

Will's joy at being with Tessa was corroded at by his parabatai's growing grief, and his beloved's slow descent into madness. They would only be whole when they were together, he knew, and the could not truly be together until Tessa could see and hold Jem once more. And he despaired, considering going blank so that the two might have the happiness they lost too early.

That was when they heard of the charm, the one that allowed a citizen to see Guardians.

That was when they found their chance.