This article is from the Tolkien Newsgroups FAQ, by Steuard Jensen sbjensen@midway.uchicago.edu with numerous contributions by others.
Given that Tolkien never firmly decided on the origin of Orcs (as
discussed in question III.B.14), it is unsurprising that little is
known about their fate. It seems clear that creatures descended even
in part from humans would be mortal (as the Princes of Dol Amroth were;
Elrond and his kin were special exceptions). In one his many
conflicting essays about Orcs (Text X of "Myths Transformed" in
_Morgoth's Ring_), Tolkien does give a direct answer to these
questions:
They could be slain, and they were subject to disease; but apart
from these ills they died and were not immortal, even according to
the manner of the Quendi; indeed they appear to have been by nature
short-lived compared with the span of Men of higher race, such as
the Edain.
The apparent clarity of this statement may be misleading, considering
the frequency with which Tolkien made and contradicted assertions about
Orcs in the "Myths Transformed" texts.
There are several reasons to believe that at least some Orcs had
very long lives. One of the strongest is based on the death dates of
the Orc Azog and his son Bolg. Azog was killed at the Battle of
Azanulbizar in 2799, while Bolg was killed at the Battle of Five Armies
in 2941. Thus, Bolg was still alive and strong over 140 years after
his conception.
More circumstantial evidence can be found in "The Choices of Master
Samwise", when Sam overhears Gorbag speaking to Shagrat:
'...if we get a chance, you and me'll slip off and set up somewhere
on our own... somewhere where there's good loot nice and handy, and
no big bosses.'
'Ah!' said Shagrat. 'Like old times.'
At this time, Sauron had been openly ruling Mordor for sixty-eight
years, but Shagrat and Gorbag seem to recall a time when he wasn't in
charge. This could mean that they were over seventy years old, that it
took Sauron many years to establish firm control over his minions, or
that they heard stories of "old times" from their elders.
Gorbag also makes reference to "the Great Siege", which might refer
either to the siege of the Last Alliance on Mordor or to the siege of
Angband in the First Age, but it is less clear that he actually
remembers it. Similarly, the goblins who captured Bilbo and Thorin in
_The Hobbit_ had a surprisingly clear memory of the swords from
Gondolin that they called "Beater and Biter", but there are
explanations for that other than personal experience.
As for their fate after death, this would depend very much on their
origins (again, refer to question III.B.14). Beasts would presumably
not _have_ a fate after death, and it seems likely that creatures
descended even in part from Men would share their Gift and leave the
world. Near the end of Text VIII of "Myths Transformed", Tolkien
comments that if there were an Elvish strain among Orcs, "dying they
would go to Mandos and be held in prison till the End." It also seems
possible that Orcish spirits would refuse the summons to Mandos, as
discussed in question III.B.6 of this FAQ; it is even conceivable that
they could be reborn or re-embodied if they then returned to Morgoth or
Sauron.
 
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