The sea monster Perseus kill called Cetus，and kraken is from Norse mythology—— don't be fooled by Hollywood films
Reverse communion is an awesome idea in the first glance, since in cbm no blood slave is needed for cast sabbath slave. But it is actually unrealistic at all: Kraken have only blood 1, so a sabbath slave will be 99 fatigue, not to mention this spell cannot be cast underwater....perhaps give it a slave matrix will make this idea into practice.
I have made some test on kraken, and seems a fully buffed kraken can defeat dozens of knight of the deeps without risk of been killed. The only problem is you need quite some meat shield to feed on those KotDs so that they will not shred kraken into pieces before it finish buffing. But it appears to be quite weak against SCs, a sea troll king with frost brand and 24 defence(27 after quicken) and less than 20 protection have roughly 50% chance to kill it, and 50% chance to be rooted rather than killed. If the sea king cast water shield and became 32 defence, then the kraken can not even touch it.