Spoiler Alert: WARNING! This page happens to contain a lot of heavy spoilers that are all originated from the Overlord author, Kugane Maruyama and his collection of written works. Anime-only watchers and Manga-only readers, please take some time off to reflect it over for a bit before choosing to expose yourself on reading the information below recklessly.
Warm Bottle is a magic vessel that acts as a thermus and heater.
Warm Bottle is a magic tool counterpart for home appliances. Princess Renner used it quite liberally when receiving a particularly important guest and rarely brought it out otherwise.
It appears like a regular teapot, the same type used for tea parties.
Warm Bottle has the effect of maintaining the temperature and quality of stored beverage for one hour. It did not hold nearly enough liquid for eight occupants.
Renner possesses one and it is considered to be part of her prized possessions.