Veterans will know: the masterstrike in Kingdom Come: Deliverance is the lifesaver in every combat. Here's where to learn it in KCD2.

Doesn't matter if you're new to Kingdom Come or already a veteran who mastered the first game, you'll quickly learn (or already know) that masterstrikes are, without question, the most important combat skill to have. Because of Henry's injuries in KCD2, he'll need to relearn it first to take on enemies with ease. Luckily, you can get it relatively early on. Here's how it works and where to get it.
Where To Learn The Masterstrike In Kingdom Come: Deliverance 2
The masterstrike is an essential sword technique that'll make combat much easier – it's unblockable by your opponents and once you've mastered it, you'll have a much easier time, even against heavily armored and armed enemies.
After your falling out with Hans, you'll meet a beggar woman who tells you about Tomcat – you'll get the quest to meet him from her. If you've missed her, don't worry: you can still head straight to your teacher.
Starting from Troskowitz, you'll want to head straight west. You'll pass the small village of Zhelejov, but you'll want to go even further west, pass the house of Herbwoman Bozhena and Pavlena and head into the woods. You're looking for the Nomad's Camp up some cliffs – the yellow marker shows where to find Tomcat, the Master to teach you the masterstrike anew:

In the Nomad's Camp, you'll find a training ground similar to other ones, and during sunlight hours, Tomcat lingers right there. Talk to him, learn some combos, prove yourself and ask him to teach you more – read: masterstrike.
New Game, New Masterstrike
The masterstrike in the first game was fairly simple – perfect block basically equals unblockable strike. Even if you missed the perfect block, you were still relatively safe as you've just downgraded to "normal blocking" and didn't necessarily take a hit.
In Kingdom Come: Deliverance 2, things have changed slightly. The perfect block is still a viable option if you're not confident enough in your combat skills yet, but the masterstrike is the unbeatable choice once you know what you're doing.
Still, it works differently this time around. It takes some practice, but once you're there, you'll be nearly unstoppable. In KCD2, there's no failsafe to the masterstrike and it has nothing to do with blocking anymore. More scary, but the payoff is worth it.
You need to pay close attention to your opponent and have an eye on where he's attacking you from. Choose the opposite direction and instead of blocking, hit him back at the same time he's attacking you. For example, if your opponent is attacking from the right, you'll want to attack from the left and vice versa.
The masterstrike is unblockable and if you've got a keen eye and good timing, you can land hit after hit after hit – super satisfying. But be careful: Henry is not the only one who's heard of masterstriking enemies to death. Make sure to never give your opponents the opportunity to use the technique against you. If he's holding his sword on his right side, don't attack from the left, but strike from above. Chances are they'll block your attack, but that's way better than getting masterstruck yourself.
It's best to let your opponents make the first move to counter them – patience is key in Kingdom Come: Deliverance. Spar with Tomcat and take on bandits with no good armor, preferably lone ones, to get a feel for the timing. Once you've mastered that, you'll basically be a one-man-army!