Colossus Climber (3.5e Feat)
From D&D Wiki
Colossus Climber [Fighter, Tactical]
Body Climb: Make a grapple attempt, with size penalties reversed for the defender (so that the larger they are, the easier they are to grapple). If successful, you move into their space. They are not considered grappled, although you are. You have cover against the enemy you are currently climbing. To remove you the enemy can take a standard action to shake you off with a Strength check, and you may make Climb checks instead of Strength checks when opposing.
Vital Strike: While climbing an opponent you can take a full attack action to attempt to strike a critical blow against them. Your critical threat range increases by +2, and you gain +4 to confirm your critical. After making a vital strike, regardless if it was a critical hit, you take a -5 penalty on climb checks to stay on for 1 round.Stop Hurting Yourself: If anyone attempts to attack you while climbing an opponent, you can choose to replace your AC by your Climb check against one attack, and the blow instead strikes against the creature you are climbing (who may be hurting itself, if it is the attacker). You can do this once a round. Afterwards, you take a -5 penalty to climb checks to stay on, for 1 round.