Ashmare Ruby

Wondrous item, Very rare (requires attunement by appeasing the fire spirit in the ruby)

This small, finely cut ruby fits easily in a pocket. Close inspection reveals a fire spirit dancing inside.

If you use an action to speak the command word and throw the ruby to a point on the ground within 60 feet of you, the ruby becomes an ashmare. If the space where the creature would appear is occupied by other creatures or objects, or if there isn't enough space for the creature, the ruby doesn't become a creature. The ashmare uses the statistics of a nightmare, except its alignment is neutral and it doesn't have the Ethereal Stride ability.

The creature is friendly to you and your companions. It understands your languages and obeys your spoken commands. If you issue no commands, the creature defends itself but takes no other actions.

The ashmare reverts to a ruby if it drops to 0 hit points or if you use an action to speak the command word again while touching it. If it drops to 0 hit points, it can't be summoned again until you complete a long rest.