Source: https://tappedout.net/mtg-decks/zombie-horde-co-op-magic/

> Gameplay Rules

The objective is for the allied team to survive and eliminate the threat of the horde.
Players achieve victory when the horde deck has no cards remaining in its library, no cards in hand, and no creatures on the battlefield.

Players will lose the game at zero life, as normal.

If the team were to attack or otherwise deal damage to the horde "player," then that number of cards are put from the top of the horde's deck into its graveyard.

> Player Rules

There are up to four players total, and each player brings his or her own Commander deck.
All players share their turn and life total, a la Two-Headed Giant, and each contributes 20 life to the starting total (one player is 20, four players is 80).

The team takes the first three turns of the game, then alternates with the horde deck.

> Horde Rules

There is no player for the horde deck; it will play automatically.
The horde is a 100-card deck, taking away a random 25 for each player under four on the team (three players is 75 cards, two players is 50).

The horde is built using token cards as well as regular Magic cards.

The horde's turn starts by revealing the top card of its library. If it's a creature token, it is set aside and this process repeats until a nontoken card is revealed. Then, all of the creature tokens are cast (they cost 0 Mana), followed by the revealed spell.

The horde will only cast spells once per turn, at the start of the turn. A creature hit by Boomerang or similar will be cast again after the revealed spell for the next turn has been cast.

The horde deck has any amount of mana needed, and can always pay additional costs required (such as Sphere of Resistance and Propaganda).

All creatures the horde controls have haste and must attack each turn if able.

If there is a choice presented for the horde, such as a player casting Fact or Fiction or controlling a planeswalker that the horde's creatures could attack, the choice is made as randomly as possible.

> Boss

The Boss is a special type of enemy, and cannot be targeted. It has the same abilities as a Planeswalker, and uses the same loyalty counters, except it can't be destroyed if it's counters reach 0.

Every Boss turn, it can use it's special abilities. If it has not enough loyalty to use its -X abilities, it will always use its +X ability.

If there are enough Loyalty counters for the -X ability, then the Boss will chose one of these abilities at random. To determine how to chose, launch a D20:
1-12: Boss use its first ability (generally +1/+2).
13-18: Boss use its second ability
19-20: Boss use its ultimate ability

If there are more than one +X or -X ability (except for the ultimate) then chose between both at random (50% chance each).

Boss will vanish whenever the Horde is defeated.

> Lieutenants

A Lieutenant is a special type of ennemy. It cannot be targeted or blocked unless there are no more creatures on the board or if all attacking creatures have been already blocked or dealt with.

It cannot be destroyed by effects that says so, but it can be dealt combat and non-combat damage if the first condition is met (no more creatures on the board or if all attacking creatures have been already blocked or dealt with).

If a lieutenant is defeated, players can take their equipments for their own use.

> Horde Deck Build Philosophy

For the Horde deck, there are a few important factors to consider when choosing your nontoken cards.
First, almost no activated abilities. These are brainless zombies, so all they need to do is enter the battlefield, attack, and then die! Effects based around these triggers will work fine. With this in mind, you also want to avoid any spells that require the Horde to choose a target. Again - brainless zombies!

Secondly, you want a good mix of "bombs" and "fizzles". This adds to the tension of the Horde's turn, never knowing if you will flip a measly Diregraf Ghoul or a dreaded Plague Wind.

Lastly, flavor, flavor, flavor! This is a fun, casual format. Make sure all of your creatures have the Zombie subtype, minus a few that may fit the theme well (for example, I added Erebos, God of the Dead, because, c'mon, God of the Dead.) Your noncreature spells can vary, but should fit into the general theme of Black Zombies.

If you find the horde too easy or too hard for your playgroup, feel free to remove / add more powerful cards to the horde as necessary. After all, this format is at its most fun when the matches are nail-biters!

> Rule Variations

In some cases, the strength of your playgroup may necessitate some changes to the rules in order to keep the maximum challenge. Here are some variations you can try if you are looking for a challenge:

For increased game tension and growth, ignore the "reveal until a nontoken is revealed" rule and instead reveal cards in increasing amounts. Turn 3 the Horde flips 3 cards, followed on turn 4 by 4 cards, and so on (these numbers can also be altered). This ensures that the deck consistently grows and remains a serious threat no matter what is flipped.

Some other variables you can alter to find your preferred difficulty: reduce the number of turns before the Horde begins attacking, increase the number of cards in the deck per player, or set special limitations such as only being able to cast your commander once. Have fun with it!

*Special note: Some cards in MTG will break this game mode fairly easily. Cards like Mind Funeral, Teferi's Moat , etc. can instantly win the game for your team or permanently lock down the Horde deck. In situations like these, there are two options - either reveal the card to your playgroup and cycle it, or decide on a reasonable house rule for that card. The fun of Horde mode come from the challenge, so it's best to avoid using these types of cards. Use your best judgment!

> Set Up

First: choose one boss from the list randomly.