What kind of damage does the warlock’s eldritch blast
deal? Does damage reduction or resistance or immunity to
energy protect against the blast?
In its basic form, the eldritch blast’s damage has no type.
Since it is a spell-like ability, damage reduction does not
protect against the eldritch blast. Since it does not deal energy
damage, resistance or immunity to energy has no effect on the
eldritch blast’s damage.
Certain eldritch essence invocations change the type of
damage dealt by the eldritch blast. For example, brimstone
blast makes the eldritch blast deal fire damage, which means
that resistance or immunity to fire would apply against this
eldritch blast’s damage (as would vulnerability to fire, such as
that possessed by a frost giant).