Some TMNT stuff really isn't for little kids.
Ra's al Ghul - real name unknown - is an over 700 year old Arabian man whose longevity is thanks to special springs located across the earth dubbed "Lazarus Pits". As a warrior who has lived so long, he is an unparalleled master of every form of combat available in his lifetime.
Ra's makes an appearance at the end of Totally Awesome Team-Up, in the Foot Clan's helicopter. As Shredder makes an escape from the Ninja Turtles, Splinter and Batman in this helicopter, he finds his Foot Ninja dead at the hands of Ra's' League of Assassins. The leader himself was in the helicopter, indicating that he is now Shredder's new partner.
He put the resources of the League at Shredder's disposal, allowing the extra-dimensional ninja to discern that the incoming arrival from his own dimension was actually Casey Jones, with several canisters of mutagen. Ra's al Ghul and Shredder took the mutagen and headed to Arkham Asylum, even as Ra's' League of Assassins cohorts converged on Gotham.
Shredder and Ra's successfully mutated all the inmates of Arkham, and pitted them against Batman and Robin. However, the tide was turned when the Turtles arrived to assist Batman, and Ra's found himself facing all four skilled ninjas in combat at once, eventually being defeated by them. Upon hearing from his grandson that the League assassins were being apprehended by the GCPD, he fled the country.
Ra's suffered another humiliating defeat when, as a result of his failed alliance with Shredder, he was considered too weak to lead the League. His place was usurped by Bane, a development that Ra's regarded with some bitterness.
After Bane was transported to the Turtles' dimension, Batman went to Ra's to obtain a part of a dimensional portal that he needed to travel there. Ra's gave it to him without a struggle, feeling that it was to his benefit if both Bane and Batman were gone.