Better Assassin

During the first round of combat, you can use your action to get the drop on a creature you can see and you are hidden from. Make a contested stealth check against the creature's perception and move undetected up to your movement speed, within your weapon's range of the target. If your stealth check succeeds you may make an attack at advantage, which counts as a critical strike if it succeeds. If your stealth check fails, the creature may immediately use it's reaction to make an opportunity attack against you, or move up to its movement speed without provoking an opportunity attack from you. If the creature is outside of your weapon's range or cannot be seen by you after resolving this roll, your attack automatically misses. If the target can be seen and is within your weapon range after this roll is resolved, you may make a normal attack against it.

You can use this feature once, and it resets at the end of a long rest.