The Golden Bough

"O goddess-born of great Anchises' line,
The gates of hell are open night and day;
Smooth the descent, and easy is the way:
But to return, and view the cheerful skies,
In this the task and mighty labor lies.

...

If you so hard a toil will undertake,
As twice to pass th' innavigable lake;
Receive my counsel. In the neighb'ring grove
There stands a tree; the queen of Stygian Jove
Claims it her own; thick woods and gloomy night
Conceal the happy plant from human sight.
One bough it bears; but (wondrous to behold!)
The ductile rind and leaves of radiant gold:
This from the vulgar branches must be torn,
And to fair Proserpine the present borne,
Ere leave be giv'n to tempt the nether skies."

- Virgil, Aeneid

 When the soul is not long gone from the body, it may be recalled with the spell Raise Dead. Once the soul has passed across the river Styx, however, the only way for the dead to be brought back to life is to stage a rescue in the depths of Hades.

Fighting all the guardians of the underworld is an almost impossible task. Those brave few who would tread the darkened way are advised instead to seek the Golden Bough - a branch of gold that  grows in a secluded grove far from civilisation. The quest for the Bough itself is not without dangers, but once one has acquired it the passage into Hades becomes easier. The ferryman Charon is bound by duty to convey anyone who carries the Golden Bough, but only if they promise to present it as a gift to Proserpine, who is queen of the underworld. 

As for the return journey - Charon will not ferry the spirits of the dead back to the land of the living. The enterprising resurrectionists must find a different path back to the surface.