Slay The Spire is a card game first and foremost, and anyone who's ever had experience with, say, Magic: The Gathering, will recall that there is such a thing as an infinite combo in virtually any card game with layers of complexity.
Slay The Spire infinite combo is, similarly, not a single move you can pull off right off the bat, but rather a type of clever card interaction which can permanently commandeer the board and win fights in mere moments.
While it's easy to wonder how to pull off infinite combos in Slay The Spire, actually managing to do one isn't all that difficult, either, provided that you learn the basics of the game well. Of course, this means knowing the cards by heart, too!
One particularly curious infinite combo turned up when a Twitch streamer by the name of RiskyCB began to heavily invest in removing cards from his deck instead of hoarding them, as you'd usually want to. Whenever he'd come across a merchant, RiskyCB would pay to remove one of the cards from his deck, so that the drawing pool eventually got reduced to only two cards.
These two cards were Flash of Steel and Dual Wield, the interaction of which allows for a literally infinite amount of damage to be dealt in the very first turn! Of course, reducing one's deck to two specific cards requires lots of luck and effort, but the fact that it is possible makes for fun theorising, if nothing else.
There's lots of possible infinite combos to pull off in Slay The Spire, and new ones are being discovered all the time. Who knows what sort of wacky interactions lie in wait as Slay The Spire nears its full release.