Slay The Spire has got an abundance of weird, offbeat relics and items that can be completely overpowered, if you've built your deck to support them. The matter of combining all the moving parts just right is what usually comes in the way, though, and this goes for the Pocketwatch, too.
Slay The Spire Pocketwatch relic is a special item that buffs the number of cards you draw, if you play less than 3 cards per turn. It's situational, but works phenomenally well in certain cases.
If you're straight-up wondering how to get a Pocketwatch in Slay The Spire, we've got bad news - you get it the same way you get the vast majority of other relics in the game: by finding them at random, or by purchasing them off the vendor. Again, random.
The Pocketwatch says that whenever you play 3 or less cards in a turn, draw 3 additional cards at the start of your next turn, which may not sound particularly enticing at first, but combined with cards such as Rampage, which does more damage the more often it's cast, it gets really destructive really fast.
In most card games, free card draw is almost always a good idea, and since there's no failure state for running out of cards to draw, this goes doubly so! Slay The Spire is set to come out from Early Access on 23rd of January - who knows what goodies the developers have in store for us then.