Colossal Grappler

You have learned to use your smaller size to your advantage while fighting a larger opponent. increase your Strength score by 1. You can now perform the grapple action on large or bigger creatures. If the grapple succeeds, instead of giving the condition grappled to the creature, you can climb up on it without spending any movement. At the start of each turn you spend on the creature you have to succeed a strength saving throw or fall off, landing prone in an unoccupied space next to the creature. If the creature is incapacitated or prone you automatically succeed the saving throw. The difficulty of the save can be seen bellow. While on the creature, all attack rolls of 15 (before modifiers) or higher count as critical hits. 

The difficulty on the strength save depends on the creature’s size:

• Large creatures give a DC of 15
• Huge creatures give a DC of 13
• Gargantuan creatures give a DC of 10