Dreadhorde Invasion

Mana cost: {1}{B}
Type: Enchantment
Set: Midnight Hunt Commander #114 (rare)
Price: $0.74 USD

At the beginning of your upkeep, you lose 1 life and amass Zombies 1. (Put a +1/+1 counter on an Army you control. It's also a Zombie. If you don't control an Army, create a 0/0 black Zombie Army creature token first.)
Whenever a Zombie token you control with power 6 or greater attacks, it gains lifelink until end of turn.

Dreadhorde Invasion has 7 printings, first in War of the Spark (2019). EDHREC ranks it #1,333 among Commander cards. It appears most in Sephiroth, Fabled SOLDIER // Sephiroth, One-Winged Angel decks (11,486 on EDHREC). 4 public decks on TCG run it. It has 9 official rulings.

Legality

commander: legal
legacy: legal
vintage: legal
pauper: not legal
modern: legal
pioneer: legal
standard: not legal
brawl: legal

Public decks that run Dreadhorde Invasion

Enduring Enchantments - Commander Masters, led by Anikthea, Hand of Erebos
Eternal Might - Aetherdrift Commander, led by Temmet, Naktamun's Will
Multiverse Reforged - Reality Fracture, led by Jace, Multiverse Architect
Undead Unleashed - Midnight Hunt Commander, led by Wilhelt, the Rotcleaver

Commanders that run Dreadhorde Invasion most

Deck counts from EDHREC.

Sephiroth, Fabled SOLDIER // Sephiroth, One-Winged Angel: 11,486 decks
Sauron, the Dark Lord: 10,712 decks
Wilhelt, the Rotcleaver: 10,535 decks
Jace, Multiverse Architect: 9,204 decks
Temmet, Naktamun's Will: 7,146 decks
Saruman, the White Hand: 3,920 decks
Braids, Arisen Nightmare: 3,873 decks
The Scarab God: 3,865 decks

Often played with Dreadhorde Invasion

Cards that show up in the same decks on EDHREC.

Rulings

2019-05-03: The power of the Zombie token is checked only immediately after it attacks. Any other abilities that trigger when creatures you control attack won't have resolved yet.
2019-05-03: If the token's power changes later in the turn after it has attacked, including while Dreadhorde Invasion's second ability is on the stack, it won't cause the token to gain or lose lifelink.
2023-06-16: Amass Orcs works the same way, except you create a 0/0 black Orc Army creature token if you don't control an Army. If the Army creature you chose isn't already an Orc, it becomes an Orc in addition to its other types. By combining cards with amass Orcs and amass Zombies, you can end up with an Orc Zombie Army.
2023-06-16: Some cards that cause you to amass Zombies also provide bonuses to "Zombie tokens." These affect any token that happens to be a Zombie, not just a Zombie Army you've amassed.
2023-06-16: To amass Zombies N, if you don't control an Army creature, create a 0/0 black Zombie Army creature token. Then you choose an Army creature you control and put N +1/+1 counters on it. If that Army isn't already a Zombie, it becomes a Zombie in addition to its other types.
2023-06-16: If you don't control an Army, the Zombie Army token you create enters the battlefield as a 0/0 creature before receiving counters. Any abilities that trigger when a creature with a certain power enters the battlefield, such as that of Mentor of the Meek, will see the token enter as a 0/0 creature before it gets +1/+1 counters.
2023-06-16: Amass abilities are now written as "amass [subtype] N." Previous cards with amass have received errata to say "amass Zombies N."
2023-06-16: Some cards refer to the "amassed Army." That means the Army creature you chose to receive counters, even if no counters were placed on it for some reason.
2023-06-16: In the rare case that you control multiple Army creatures (perhaps because you played a creature with changeling) while you amass Zombies, you choose which of your Army creatures to put the +1/+1 counters on. If that creature isn't a Zombie, it becomes a Zombie in addition to its other types.