Mechanics
Main Zones
In Play
Player HP: Each player begins the match with 25 HP. When a player's HP reaches 0, they lose the game unless they Can't Lose due to a card effect. There is no upper limit to how much HP that can be obtained.
Board Space: Players can have up to 10 units in play at a time. When a new unit would be played or a level up reward would enter the board, the player Chooses a unit to replace with the new unit. In all other cases, such as creating cards or moving them, the unit goes to the graveyard.
Hand
Handsize: Players may have up to 13 cards in their hand at any time. When a player would have more than 13 cards in their hand, the extraneous cards are discarded immediately. At the end of the turn, if you have more than 10 cards in your hand, you Choose cards to discard until you have 10. This happens before end-of-turn triggers.
Mulligan: At the beginning of the game, each player is shown four cards. They may choose any number of cards from those four to be randomly redrawn, then keep the resulting four cards as their starting hand.
Deck
Decksize: Players may put a minimum of 45 cards and a maximum of 300 cards into their deck. You can have up to 3 copies of a single card in your deck.
Fatigue: When a player would draw from an empty deck at the beginning of turn, they instantly lose the game.
Mana System
The mana system is displayed as available mana/max mana on the lower left section of the board.
In order to play a card, players must pay its mana cost as displayed in the upper right circle. They spend their available mana to do so. At the start of each turn, available mana is replenished up to the amount of max mana indicated. Players may increase their available mana above their max mana amount using card effects such as Courier Knight.
Each player begins the game with 1 max mana. At the start of each turn, if the player's max mana is below 13, it is increased by 1 automatically. If a player has 13 or more max mana, it will no longer be increased automatically, but they may still increase it using card effects such as Westwind Channeler.
Heroes/EXP system
See Heroes.
Turn Structure
Collective utilizes a simultaneous turn structure. Each turn has two phases, the Main Phase followed by the Combat Phase. At the start of the turn, each player draws 1 card, gains 1 EXP, and gains 1 max mana (if the cap hasn't been reached).
Main Phase
During the main phase, card and ability resolution happens. At the start of the game, a random player is assigned initiative, which then alternates between players each turn. Players simultaneously choose which cards to play, which activated abilities (actives) to use, and the order in which they resolve. Those decisions are hidden from the opponent, and the effects don't happen immediately, but go on a stack where they wait to be resolved, until both players finalize their decisions by pressing the done button. Then, starting with the player with initiative, all cards/abilities are resolved in the order they were selected in. After all of the effects of the cards/abilities of the initiative player resolves, then all of the off-initiative player's actions resolve. This ends the main phase.
Example
The off-initiative player has a Test Post in play from the turn before. During the Main Phase, they play a Rabock Helmet on the Hero. The on-initiative player plays a Fireball on the Hero. After both players lock in their choices, the on-initiative player's cards resolve first, and their Fireball destroys the Hero. The off-initiative player's cards then resolve, and without a unit to target anymore, the Helmet automatically goes to the discard.
Combat Phase
During the combat phase, unit combat happens. Players again make simultaneous decisions that don't take effect until both have confirmed their choices. The combat system utilizes blocker's choice, meaning that attacking units do not have target selection and automatically attack the enemy player if unblocked, while blocking units can block any single attacking unit of their choice (multiple blockers can block the same attacker). Each player first select the units they wish to attack with. After attackers are locked in, defenders are then selected. After defenders are locked in, combat happens. If any unit is blocked by multiple units, its controller Chooses what order that unit attacks its blockers. The on-initiative player's attacking units deal damage first, in order from oldest to newest, then the off-initiative player's attacking units deal damage. This ends the combat phase.
In combat, each unit deals their attack in damage to the health of the unit they are combating. This damage persists through each turn. The attack value of an attacking unit is dealt cumulatively to each defender, not the same amount to each, and will be dealt even if the attacker's health goes to or below 0 during combat.
Example
The on-initiative player attacks with a Lizabo Hunter. The off-initiative player blocks first with a Sunflower, then a Spooky Spider. The Lizabo Hunter deals 3 damage to the Sunflower and no damage to the Spooky Spider, while the Spooky Spider kills the Lizabo Hunter with Deadly. Had the off-initiative player blocked in the opposite order, the Spooky Spider would've been killed.