A win condition (occasionally shortened to win-con) is an informal game term. Your deck's win condition is your plan to win the game, both when you're building the deck and in the middle of a game. You should always keep an eye on your plan to win, even if it might change later.
The basic ways to win this game are for your ring-bearer to survive to the start of the Regroup Phase at site 9, for your opponent's ring-bearer to die to wounds or being overwhelmed, or for your opponent's ring-bearer to be corrupted. The last generally only happens if the ring-bearer's resistance reaches zero, although there are a few cards that corrupt the ring-bearer under other circumstances.
While there are only three basic win conditions, usually the emphasis is more on which of them you're focused on and how you plan to execute it. Your Free Peoples deck might be more focused on playing conservatively and surviving (and might even help your Shadow deck cycle or filter) or more focused on becoming very strong and running down the site path. Your Shadow deck might plan to swarm your opponent and overwhelm their ring-bearer, stack up burdens to corrupt the ring-bearer, simply beat down whatever companions your opponent plays, or, again, support your stronger Free Peoples side by cycling and filtering.
Some decks might bring a specific card or combo of cards as a backup or alternate win condition. For example, a Moria Shadow deck might be focused on winning with a traditional swarm, but also bring Lost to the Goblins (1R189) to occasionally catch the opponent unawares.
Automatic Corruption Cards
A small handful of cards automatically corrupt the ring-bearer regardless of resistance, and are occasionally referred to as win condition cards. However, all of them are only rarely played.
- Despair (1R243)
- The Irresistible Shadow (1R252) - also reprinted in another culture as The Beckoning Shadow (12R83)
- Held (6R109) - the only Free Peoples card that does this
- Put Forth His Strength (7R205)
- Shapes Slowly Advancing (11R217)