So here goes.
Starting with the abilities that target enemies, on the right hand side of the skill tree.
Warcry is an entry point for protection and debuffs in this tree. It is a way to dramatically increase your characters threat level and is an active skill.
Mocking Blow requires Warcry, and is an active skill that will debuff your target. It lowers their Attack Strength, Defence and Armor stats for a reasonably brief period of time, though enough to get a few attacks in most likely.
Disarm requires Mocking Blow, and extends the debuff by debuffing the attack strength, and pretty significantly reducing the attack damage that an enemy will perform.
Intimidate requires Disarm and is another debuff that will drop an enemy's Attack, Defence and Armor stats, while also decreasing the damage they do on successful hits.
All these abilities can be stacked onto a single creature, though they will take some time to apply by a single warrior. The same effect, from different warriors will NOT stack, but there's nothing stopping warriors working together to apply different debuffs as a team.
Next up, Shield Wall, an ability activated in combat to give the warrior a buff that gives them a good buff to their defence temporarily, gives them a fantastic buff to their armor, but makes attacks much weaker for a period. Best to use once you have a creature firmly fixated on this warrior to make it easier for healers to keep up.
Intercept is an ability that requires Shield Wall, but enhances the protection that a warrior would give themselves to an ally. The warrior pulls an ally back and inserts themselves into that location instead - so best used when you're jumped and the delicate types end up on the front lines of combat, or to quickly get to a location that is blocked by creatures, friendly or not, in your way.
Defensive Stance is a passive ability that will give you a permanent buff to your Defence, and also y9our armor stats.
Polished Armor, requires Defensive Stance, and builds on this defensive mindset for your character, giving them a passive buff to their defence and armor stats.
Lastly, Chaotic Calm is a passive skill that the seasoned veteran of numerous battles can eventually attain. It requires both Polished Armor and Intercept Skills, but delivers growing buffs highlighting the perfectly attuned calmness that only the most veteran and composed fighters can display. Your character's damage will go up, their attack strength will go up, and both Defence and Armor stats will go up.
While I am happy with all these skills, both in their organisation in the tree, and also roughly the order they can be unlocked in, I am leaving it open to tweak just how good each one is. Should Disarm reduce damage by 15%? 40%? How much extra armor should Defensive Stance give? Is an additional percentage right, or should it be a flat number that doesn't scale? I think there's a lot there to tweak to get right, but that's going to be a fun process when the fights get more complex and more players are in the game, running about and trying to get the best outputs from all their characters!