Hmm this is a good one.  Unstable affliction and shadow word death first comes to mind but both would do damage instantly and not after 18secs and dispel isnt a shadow spell. :(

Shadow reflector could be used but priest doesnt have a shadow spell that does delayed damage like that.

Damn was thinking maybe levitate > fall damage but that isnt shadow either. :(

Maybe some item use.  Well Im stumped, Ill try and give it another go b4 24hrs are up or someone else answers 1st.

Ruddy good question though. :)

Lol, dont suppose its a trick question and he is underwater?^^