Eternal Blade can be entered at level 10, and its level 10 ability is pretty darn game breaking. It grants an extra turn as an immediate action (meaning it can be used at any time during the round, even when its not one's turn). The other abilities granted by Eternal Blade are also pretty cool. It's a full BAB class and grants maneuvers as a Crusader, but has access to Devoted Spirit, Diamond Mind, Iron Heart, and White Raven (the schools with the best maneuvers, like Time Stands Still, Iron Heart Endurance, Strike of Righteous Vitality, White Raven Tactics, and White Raven Hammer to name a few off the top of my head).
You have to stay in it for all ten levels for its best ability and it can be entered at level 10. The biggest requirement is 10 BAB (which isn't a problem for a Crusader/Fighter). Technically one has to be an elf, but most DMs don't care about it. There are some classes which work better than Eternal Blade, but the class is good in lots of situations.