The Ninja Master is one of the few instructors available that is not a guildmaster. He is extremely difficult to find due to the fact that he wanders the mud but can normally be found in areas close to Midgaard. Once found, it is highly recommended that you follow him as he will continue to wander the mud.
He teaches mainly skills for thieves such as climbing, disguise, retreat and spy but also teaches disarm doorbash and switch which can be learned by warriors. The riding skill he teaches can be learned by any class. His expertise comes at a price of 1-3,000 gold depending on your charisma per practice and you can only practice the skills a few times.
The ninja master can be killed but it is highly rrecommended that you NOT attempt to kill him as he may be the one that kills you and also since he wanders the mud it isn't for certain if he repopulates or not so it's better to leave well enough alone in that regards and only leearn from him if you manage to find him. I hope this information was helpful and enjoy Criterion Mud!