So cthulu is guaranteed to drop one of the following items. See the wiki for details on each of them.
Frostfire
Trickster's Mischief
Gae Bolg
Full Moon
Heavens Fury
Short Circuit
Goka Uln

He also has a chance to drop one of any dreamland specific item. I would assume he could also drop puzzle boxes and cthulu set pieces, but I have never seen it happen. See the link below for dreamlands loot table.
http://annihilus.net/showthread.php?t=8111

Lastly, he has a chance to drop effects that are not available at Hadriel