Summon weapons

A Summon weapon is a weapon that spawns a helpful secondary "Minion" character that follows the player, aiding during battle by automatically attacking enemies within range. Minions cannot be damaged or killed, and follow the player indefinitely until the player dies, summons a replacement Minion, or cancels their buff. There are several different Minions, some of which can walk or fly, and attack via melee or ranged means. A Minion costs 10 mana to summon initially, but follows the player indefinitely thereafter at no further cost. Although they cost mana initially, Minions deal summon damage, rather than magic damage.

By default, a player can only have a single Minion summoned at once. Various boosts can raise the player's Minion capacity, allowing multiple simultaneous Minions. The maximum number of minions achievable using this method is 11, using Stardust armor, Papyrus Scarab, Necromantic Scroll, Pygmy Necklace, Summoning Potion and the Bewitching Table. With Minion cap boosts in effect, the same Minion can be summoned multiple times, or several different Minions can be summoned. Minion cap boosts do not affect Pets or Light Pets.

Minions follow players in a manner similar to Pets, flying and passing through blocks as necessary to keep pace with the player's movements, though most cannot move through blocks for the purposes of attacking. Exceptions are minions summoned by the Tempest Staff, Xeno Staff, and Stardust Dragon Staff.

Sentries are exceptions to the above: While they similarly deal summon damage, they are temporary, cannot move at all, and do not count toward the Minion cap.

Sentry-style
Sentries are unique in that they are temporary and don't follow the player. Instead, they remain stationary until their time is exhausted. They don't count toward the player's minion cap, so traditional minions will not be affected by summoning a sentry. Multiple sentries can be summoned at once, but not multiples of the same kind.

Achievement
You and What Army? is the achievement you earn for having nine minions summoned at the same time.