Thats quite simple:
A crypt on level 1 summons him at level 1.
A crypt on level 2 summons him at level 3.
A crypt on level 3 summons him at level 5.
A crypt on level 4 summons him at level 7/8 (I am not sure).
A crypt on level 5 summons him at level 10.
The only problem is that Karsh himself can't give experience to the crypt. By the way: you upgrade the crypt while killing enemy units with normal barrow wights.
(And sorry for my english

)
Gruß
Melkor Bauglir