|A mighty efreet, captured by some wizard and bound into a bronze flask. Breaking the flask's seal will release it to wreak havoc - possibly on you.|
A bottled efreet is a one-time use evocable item which releases an efreet armed with a scimitar of flaming. Whether this efreet decides to serve you or attack you depends on your Evocations skill and your choice of god. Worshipers of any good god (Zin, Elyvilon, or the Shining One) always summon hostile efreets. Anyone else has a 50+(5×(Evocations/3))% chance of summoning a friendly efreet:
While having an allied efreet can be useful, be aware that it will create a trail of flame clouds behind it. This will be trouble for any other allied monsters you have following you, particularly undead monsters without regeneration, or fire-vulnerable ice beasts. Since it's an actual monster and not a summon, it may be better simply to kill it for free flaming weapon it carries (they make excellent anti-hydra weapons).
Usually, hydras will kill your allied efreets quickly in melee, as the demons have low HP and don't blink. You can collect the weapon later if you manage to lure the hydra away or kill it yourself.
Bottled efreets were removed in 0.14.