The following actions are available:
Help an ally back up You can expend a bonus action and 15 feet of your movement to help a conscious ally back on their feet. This uses their reaction. The target cannot be a size category larger than, currently unable to move or encumbered.
Attack an unsupecting enemy Any attack against an enemy who is not expecting an attack and is either unaware of your presence or is incapable of moving is made with advantage (as per RAW) and is also a critical hit.
Called shot When you take your attack, you can choose to get disadvantage and up to a -5 penalty on the attack in return for targeting a specific part of the target. This attack deals half damage, but can inflict powerful debuffs.
The following abilities are available to some enemies:
Cutthroat You can expend an action to slit the throat of an ally to immediately make them fail all their Death saving throws
Singular Strike If you possess multiple attacks you can use your regular action, as well as your bonus action and half your movement to perform one single attack of extraordinary power. The power of this singular strike is the strength of a single of your regular attack in addition to half of your regular attack for each extra attack you forego. This attack can break shield, armor and even magical items depending.