Ressucitando tópicos
Eldritch Cannon
At 3rd level, you learn how to create a magical cannon. Using woodcarver's tools or smith's tools, you can take an action to magically create a Small or Tiny eldritch cannon in an unoccupied space on a horizontal surface within 5 feet of you. A Small eldritch cannon occupies its space, and a Tiny one can be held in one hand.
Once you create a cannon, you can't do so again until you finish a long rest or until you expend a spell slot of 1st level or higher. You can have only one cannon at a time and can't create one while your cannon is present.
The cannon is a magical object. Regardless of size, the cannon has an AC of 18 and a number of hit points equal to five times your artificer level. It is immune to poison damage, psychic damage, and all conditions. If it is forced to make an ability check or a saving throw, treat all its ability scores as 10 (+0). If the mending spell is cast on it, it regains 2d6 hit points. It disappears if it is reduced to 0 hit points or after 1 hour. You can dismiss it early as an action.
Bem claro - ele não é uma criatura. Não é um constructo.
Sobre os saves
Interacting with Objects PHB p185
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Characters can also damage objects with their weapons and spells. Objects are immune to poison and psychic damage, but otherwise they can be affected by physical and magical attacks much like creatures can. The DM determines an object's Armor Class and hit points, and might decide that certain objects have resistance or immunity to certain kinds of attacks. (It's hard to cut a rope with a club, for example.)
Objects always fail Strength and Dexterity saving throws, and they are immune to effects that require other saves.
When an object drops to 0 hit points, it breaks.
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Então objetos falham saves de força e dextreza sempre - se eles forem obrigados a fazer.
A fireball afeta criaturas. Objetos não são criaturas. Logo, ele não é afetado e não precisa fazer o save.
A descrição da fireball entretanto diz que objetos inflamáveis pegam fogo. Se fosse um barril, uma casa de madeira, UMA FLORESTA, ela pegava fogo. Um objeto mágico não pega fogo.
Obejetos são imunes a grapple e restrained, já que eles são imunes a todas as condições. Isso não quer dizer que ele não possa ser movido. Como é um objeto, ele simplesmente pode ser interagido. Alguém pode pegar ele e colocar em algum lugar, que é a ação de interagir com objetos.