Ascetic Stalker( Complete Scoundrel, p. 73)
You have practiced a secret technique that combines your considerable talents in unarmed combat with a greater understanding of your inner ki.
Ki power, ki strike (magic),
Your monk and ninja levels stack for the purpose of determining the size of your ki pool. For example, a 4th-level monk/2nd-level ninja with this feat could use her ki powers a number of times equal to 3 (half the sum of her monk and ninja levels) + her Wisdom bonus (if any).
Your monk and ninja levels also stack for the purpose of determining your unarmed strike damage, as well as your ki strike class feature. For example, a 4th-level monk/6th-level ninja would deal 1d10 points of damage with her unarmed strike, and her unarmed strike would overcome damage reduction as a lawful magic weapon (as if she were a 10th-level monk).
In addition, you can multiclass freely between the monk and ninja classes. You must still remain lawful in order to continue advancing as a monk. You still face the normal XP penalties for having multiple classes more than one level apart.
A monk can select Ascetic Stalker as a bonus feat at 1st, 2nd, or 6th level instead of one of the other feats indicated for those levels (PH 41).