Siegeball

The Game

Siegeball is a game played between two teams of five players each. Each team has a tower to defend from the other team. Players can't attack the towers directly, however. Instead, there is a single ball that can be used to knock the towers down. A team loses when their tower falls.

The Arena

The game is played in a walled, rectangular arena. Normally, this arena is 125 feet long by 45 feet wide (25 squares by 9 squares), though arenas of all sizes and shapes exist. Each team's tower, a 5-foot- diameter pillar made of wood or loose stone, is set up 15 feet from each end of the arena lengthwise. Towers have 15 hit points and can only be damaged by the impact of the ball.
At higher tiers of play, the tower has more hit points, increasing by 20 hit points when the players reach 5th level (35), 11th level (55), and 17th level (75).

The Ball

Central to the game of siegeball is the ball, an especially dense sphere of solid rubber or wrapped leather that is pursued by the players. The ball is astoundingly heavy, and players move it by attacking it. Though some arenas have players strike the ball with their hands or feet, players are usually given specially crafted bats for the tasks. In especially dangerous arenas, players can be given warhammers or mauls.
The ball has AC 8 and is immune to poison and psychic damage. It also has a pool of momentum points (described below), which determine how fast it is moving.

Attacking The Ball

When you attack the ball, you can choose to hit it in one of the eight cardinal directions. Instead of dealing damage to the ball on a hit, you give the ball a number of momentum points equal to half the damage dealt, rounded down. As the ball moves in the chosen direction, it subtracts one momentum point for every 5 feet it moves until it stops. The ball can collide with walls, players, or towers.

Colliding With A Wall

If the ball collides with a wall, it bounces off at an angle corresponding to its direction and continues to move until it stops.

Colliding With A Player

If you are not prone, you can use your reaction to hit the ball if it collides with you. To do so, make an attack roll targeting the ball. While moving, the ball has a bonus to its AC equal to its current momentum points. On a hit, roll damage as normal, remove the ball's current momentum points, and send it in a new direction. If you miss, the ball hits you, and you take damage equal to the ball's current momentum points and are knocked prone.
If you don't use your reaction to stop the ball, it rolls past you.

Colliding With A Tower

If the ball collides with a tower, it deals damage to the tower equal to the ball's current momentum points and then stops.

Playing The Game

The game begins with a tip-off in the center of the arena. One player from each team makes an attack roll against the ball. The player with the higher attack roll successfully attacks the ball. Players begin in locations of their choice that are further than 10 feet from the ball and closer than 30 feet from the ball on their side of the arena.
Players act in initiative order. If preferable, the GM can choose for the teams to take turns acting, instead of keeping track of the players individually. The game ends when a tower is reduced to 0 hit points.

Siegeball Actions

In addition to attacking, dodging, shoving, and dashing, (and occasionally using an item forbidden by the referees), there are a few special actions that players of a siegeball game might take.

Block

You dig in your heels and hold your ground. You have advantage on any ability check or saving throw you make to resist being moved against your will, and you can use your reaction to attack any ball that moves within 5 feet of you. Additionally, if the ball is within 5 feet of you, creatures you choose have disadvantage on attack rolls against it.

Follow

When you take this action, choose a creature you can see within 10 feet of you. When that creature moves, you attempt to follow behind it, moving up to your speed, while remaining the same distance between yourself and the lead creature for the entire duration of the movement.

Retire

When you take this action, you remove yourself from the field and sit out the rest of the game. This may be useful to free yourself from danger, but you can no longer help your team. You can use this action to retire an unconscious player within 5 feet of you.

Tackle

Performing a tackle is a special variant of shoving a creature, and can be taken in place of any attack you make with the Attack action on your turn. You make a Strength (Athletics) check with advantage contested by the target's Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses which check to make). If you succeed, both you and the target are knocked prone in the target's space.

Attack A Player (Illegally)

In most games, an attack can only be made when the referee isn't looking. When you make an attack against another player, make a Dexterity (Stealth) check or a Charisma (Deception) check, contested by the referee's passive Perception score (usually 14). On a success, you attack the player without consequence. On a failure, you are removed from the game for the following round, and reenter the game on a side of the arena near your tower. Shoving, tackling, and grappling don't count as attacks on other players.
In the cutthroat "Everything Goes" variant of siegeball, all attacks are legal and direct assaults against other players are even encouraged.

Cast A Spell (Illegally)

When you cast a spell in an arena that doesn't allow magic, you can make a Dexterity (Stealth) check or a Charisma (Deception) check, contested by the referee's passive Perception score (usually 14). This roll has advantage if the target is within 5 feet of you, the effect of the spell doesn't include any large audio or visual cues (like certain illusion spells or spells like gust of wind), or the spell is cast without any components, such as via the sorcerer's Subtle Spell Metamagic. It has disadvantage if the target of the spell is more than 5 feet away from you and the effect of the spell has obvious audio or visual cues. On a success, you cast the spell without consequence. On a failure, you are removed from the game for the following round, and reenter the game on a side of the arena near your tower.
Generally, magical abilities that don't involve casting a spell, such as a paladin's Divine Smite, don't count as casting a spell. Inborn supernatural abilities like a dragonborn's Breath Weapon may or may not count as casting a spell, at the referee's discretion. Many more types of magic will be allowed in the "Limited Magic" variant of the game, and practically all spells and tactics are allowed with the lethal "Everything Goes" rules.