War Table

Activate it once and you keep the extra sentry slot until you die or leave the world. The buff does not care whether the table is still on the ground, so you can set it down, use it, mine it back up and carry it to the next fight. In practice it lives in your inventory and gets planted for ten seconds before anything serious.

The drop is far more generous in the first tier of the Old One's Army, and the first tier disappears the moment you kill a mechanical boss. Grabbing it early also makes the rest of the event easier, because sentries you summon yourself are not bound by the Etherian Mana budget that the event's own sentries have to respect.

The buff applies to any sentry at all, so summoners keep using it long after the Tavernkeep's crystal is packed away. It also counts as the table in a house, though it is the bulkiest piece of furniture in the game, so plan the room around it if you want one permanently placed.