What happens when a summoned demon goes out of range?

Question

The spell Summon Greater Demon is quite clear on where your summoned demon appears at.

The demon appears in an unoccupied space you can see within range...

The wording of this spell is very similar to the rest of the spells that summon things in regards to where the summoned creature materializes at. And the spell goes on to say how the demon can disappear: zero hit points, the PC loses concentration, etc.

What if you aren't using the spell in combat and you want it to fly to a neighboring city and kill someone? Are there rules for this that aren't mentioned in the spell description?

What happens when a summoned demon goes out of range?

Answer 1

Nothing interesting happens.

The “range” of the spell tells you what spaces are eligible for summoning the demon:

The target of a spell must be within the spell's range.

After this, there is no range related restriction on where the demon can go:

Once a spell is cast, its effects aren't limited by its range, unless the spell's description says otherwise.

Compare to the spell mage hand:

The hand vanishes if it is ever more than 30 feet away from you

Mage hand ends when the hand goes out of range because that’s written in the spell description. Summon greater demon has no such restriction.

Answer 2

Range is only relevant when the spell is cast

The target of a spell must be within the spell's range.

Once a spell is cast, its effects aren't limited by its range, unless the spell's description says otherwise.