Aggro

Aggro is an invisible number on your character that makes enemies believe you are nearer or further away than you really are. Positive values pull attention toward you, negative values push it off. Almost every item whose tooltip mentions enemies being more or less likely to target you is moving this single stat.

It only ever decides between players, so in singleplayer most of it does nothing at all. There is no aggro range and no leash either. An enemy with no other option targets you regardless of distance.

Most enemies pick the closest living player, and the distance they compare is taxicab distance in pixels, horizontal plus vertical, measured between hitbox centers. Your aggro is subtracted from that figure before the comparison happens.

So a skeleton standing 8 tiles from one player and 10 from another goes for the nearer one. Give the far player 100 aggro and the same Skeleton now reads that player as 60 pixels away rather than 160, which beats the other player's 128, and it switches targets. Sixteen points of aggro is one tile of apparent distance, which is the conversion that makes gear numbers readable: 400 aggro is worth 25 tiles of pull.

Old One's Army enemies run it in two stages instead. First they discard any player more than 200 pixels away using true straight line distance with no aggro applied, so aggro gear cannot pull a wave from the far side of the arena. Only among the players inside that 12 tile radius is aggro then subtracted. Finally they weigh the winner against their distance to the Eternia Crystal and go for whichever is closer.

One part of aggro does work in singleplayer, and it is subtle. If your aggro is negative and you are not using an item, most ground enemies stop turning to face you. A skeleton walks straight past and keeps going instead of pivoting to follow.

Only the sign matters, not the size, so one cheap negative accessory triggers it as completely as a full stealth build. The catch is how narrow it is. The moment you use anything, the item animation restores normal behavior for its duration. Bosses ignore it entirely. So do enemies with no left or right to speak of: things clinging to walls, swimmers, stationary casters, burrowers. And even affected enemies still jump, still fire projectiles, still perform scripted lunges. It helps you cross a corridor. It will not help you fight in one.

The Invisibility Potion does not add to your aggro, it sets it. Minus 750 while you are idle, minus 250 while you are using an item, and only when your current value is above that. If your gear already puts you lower, the potion contributes nothing.

Three accessories at 400 each, say an Arcane Flower, a Recon Scope and a Stalker's Quiver, already total minus 1200, so drinking the potion is wasted. Going the other way, a full Solar Flare set's positive 900 is erased completely by one sip, because the potion overwrites rather than offsets.

Stealth modes are evaluated last, after the potion, so those do stack on top of it. A Putrid Scent with the potion is still only minus 750, but add Shroomite stealth and you reach minus 1500. Only one stealth mode can be active at a time, and the Psycho Knife takes priority over the other two.

The floor is minus 2800, from Putrid Scent, Arcane Flower, Recon Scope, Stalker's Quiver and Vortex stealth, which is the single largest source in the game. The ceiling is positive 2200, from Solar Flare pieces, a Beetle Shell, Flesh Knuckles, a Berserker's Glove and a Hero Shield.

In tiles that spread is over three hundred, which is far more than the width of any fight. Two players at opposite ends of the scale will never split a group of enemies. Everything goes to the one wearing the positive gear.

In a group, this is how a designated tank works. Put the positive aggro on the character carrying the defense and the damage reduction, put negative aggro on the ranged and magic characters, and the fight stops rotating targets every few seconds. Since the tank pieces are also defensive pieces, the build mostly assembles itself.

Solo it is close to dead weight. The only positive aggro effect that does anything in singleplayer involves bees, which can target other enemies and can be talked into coming after you instead. Negative aggro solo is worth exactly the chasing quirk above and nothing more.

Two edge cases worth filing away. The Royal Gel is a targeted override: affected slimes treat you as a thousand pixels closer than you are and chase you regardless of your aggro, which turns it into a slime magnet rather than a slime repellent. And the Confused debuff flips an enemy to face the opposite of its current direction, so on a low aggro character who has already stopped being tracked, confusing something can point it back at you.