If a god is invincible or not actually depends of the setting, some are, some aren't. The D&D book Deities and Demigods does a very nice job of explaining the nature of divinity and how it can vary according to setting. Here are a few exerts from said book regarding various divine status. Oh, and yes we're playing in the Third Age or TA.
Gods Can Die
In the first option, gods may be immortal but vulnerable. They live
forever, if left alone to do so. If player characters can kill the gods,
then so can some nonplayer characters, such as other gods and very
powerful infernal creatures. If the divine spark can be stolen and if
divine power is limited, there's strong motivation for gods to kill
each other to gain power. Granted, your gods may not be motivated
by the desire to gain power, but other creatures may be, particularly
evil-aligned creatures and characters. If divine power is limitless or
cannot be stolen, then there's far less impetus to commit deicide.
This option allows for change in the pantheon. It also allows
pantheons to replace each other as civilizations rise and fall. If you
decide that gods depend on worshipers for power, then something
must happen when gods lose so many worshipers that they
become powerless and forgotten.
Frequent turmoil in your pantheon may lead to fewer people
worshiping any gods at all. If the gods don't endure longer than
mortals, mortals may not see the point of holding them sacred.
There's certainly no point in going through the initiation rites for a
mystery cult if the cult won't have a patron by the end of the
process. Clerics and paladins may choose to follow something
other than one or more gods if they can't count on their patron
being alive in the morning. Even if your gods can die, they should
not do so often. Divine death should be a momentous event in your
campaign. Consider the consequences of domains shifting to a god
with an opposed alignment, for instance.
If your gods can die, you need to decide where they go. Perhaps
their corpses drift through the Astral Plane. They might simply dissipate,
or you could create a plane just for the divine dead.
A monotheistic system in which the single god can die runs the
risk of, someday, being a universe without a divine being. Some
natural mechanism may prevent this from ever happening by automatically
elevating some worthy mortal to divine status if the current
deity dies. A dualistic system loses its central balance if either
deity dies. The remaining deity must immediately appoint a
worthy opponent in order to keep the universe functioning and
the balance intact. Animistic systems believe everything is alive,
and this life stems from the spirits. If the spirit of a place or object
dies, the result is an abomination. A dead place or thing is a source
of horror and perhaps fear for the faithful, even if the spirits are
hostile or feared. Being able to kill a spirit implies that the killer
could someday murder the world. Accidental death should never
be possible for spirits. Anyone intentionally killing a spirit faces a
lifetime spent in hiding from frightened faithful seeking to bring
the killer to justice. Ancestor worship is a viable alternative to worshiping
dying gods.
Gods Cannot Die
Truly immortal gods fear neither mortal nor fellow god. This doesn't
mean the same gods have always existed: New gods may come into
existence as new civilizations and faiths rise, and old ones may disappear
as civilizations fall and faiths fade. Immortality may stem from
an inability for any nondivine creature or character to reach the gods,
or the gods may be invulnerable as well as immortal. Truly unkillable
gods can provide stability and order in your campaign as characters
take comfort in the enduring existence of their gods. True immortality
boosts the faith of clerics and paladins, and makes them more
likely to commit their lives to deities. This concept is so common
that it makes little difference what sort of system it's applied to.
