Behemoth Hunter (5e Subclass)
From D&D Wiki
Your desire to prove yourself against those bigger than you towers over your limits.
- Size Tactics
Beginning at 3rd level, you have learned to use another creatures size against them. you have advantage against Dexterity saves triggered upon you from creature that is a size rating larger than you. Additionally, if the effect of the Dexterity save allows you to take half damage on a pass you take none instead, however you still take the full damage on a fail.
At 7th level, the bigger they are, the louder they scream... when a large or larger creature is targeted by your attack and it is a critical hit, the creature loses half its movement speed and has disadvantage on Dexterity saves/checks until it finishes a short or long rest.
- Greater Strength
At 11th level, strength is measured differently in your eyes. when a large or larger creature misses an attack, you can either make an opportunity attack against them, or use your reaction to knock them prone.
Beginning at 15th level, Hard hits leave openings for you to exploit. while you are holding a weapon or shield and are hit by an attack, you can make an attack with your reaction made with advantage. If the attack on you was a critical hit, this attack fails.