The conjure elemental spell says:
You call forth an elemental servant. Choose an area of air, earth, fire, or water that fills a 10-foot cube within range. An elemental of challenge rating 5 or lower appropriate to the area you chose appears in an unoccupied space within 10 feet of it. For example, a fire elemental emerges from a bonfire, and an earth elemental rises up from the ground. The elemental disappears when it drops to 0 hit points or when the spell ends.
Does this mean a creature can not be summoned unless it belongs to one of the four elements? For example, can I summon a flail snail or invisible stalker with conjure elemental?
Best Answer
Flail Snail and Invisible Stalker are both valid choices for conjure elemental
Firstly, the Flail Snail is CR 3 so it is a valid choice for this spell. The Invisible Stalker is CR 6, which would be invalid, but the spell can be upcast to increase the CR range, so if the spell is cast with a 6th level spell slot or higher, the Invisible Stalker then becomes a valid choice too.
From the PHB, pg. 225:
Secondly, both of these creatures are "elementals" (which is one of the creature types like "humanoid" or "beast"); this is what the spell is referring to when it says:
not just those that are called "Elemental" such as "Fire Elemental", etc.
This creature type is outlined in the Monster Manual, pg. 6:
Also, both of the creatures you mentioned are each associated with one of the four elements. Flail Snails are associated with earth (Volo's Guide to Monsters, pg. 144):
And Invisible Stalkers are associated with air (Monster Manual, pg. 192):