Pretext
Since this is a card game, combat can be complex to figure out initially. I've written a quick guide below on what it would look like if you're in combat. :)
Starting combat
To start combat, you will need to go up to an NPC that spawned in the dungeon, and interact with them.
Wagering
As soon as combat starts, you are prompted to wager a card.
If you were to lose the match, you lose the card you wagered to your opponent. This is true for the opponent as well since they wager a card from their deck as well.
Combat is turn based
You, your opponent, and the Sacrifice will each have a turn to place spells/move around. You can see who's turn it is by checking the "turn card" above the Sacrifice's health bar. The leftmost card is the person who's playing.
Your turn
Once your turn starts, your hand/deck will move to the middle of the screen. You can pick one of the cards from your deck to place, or you can skip the turn.
Placing spells
If you've picked a spell that you want to cast, you can freely move around the map to place that spell. Spells cover a 3x3 area, and requires the centre of that spell to at least be walkable for the Sacrifice. You won't be able to place a spell if it glows red!
Powering spells
When a spell is placed on the battlefield, it needs to be powered. You can power the spell by placing the item it requires (either shown on the spell's card or on the placed spell with a "?") next to it.
Spell triggering
Spells get triggered when the Sacrifice steps on one of the green zones, or the centre of the spell. A spell will instead fizzle if the Sacrifice steps on a red zone.
Winning (or losing)
The game ends as soon as the Sacrifice has no health remaining. The person (you or your opponent) with the most points, wins the game and gets the card the other person wagered. Did you get a tie in your match? Then both of you lose!
When an opponent happens to die, they will be "knocked out" for a few minutes before respawning and being able to fight again!