The issue is that it is five mana total to play and use the Rat and as such it is not really adding or receiving much value from something like a Bloodghast
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Since Omnath's ability is a 1st, 2nd, 3rd type ability, if you bounce him it resets
If the result is 1-14, you get to create a token of the Creature that isnt legendary, is tapped and attacking, and that gets exiled at the end of combat
Use Saw in Half on Cormela, Glamour Thief
You can easily discount it to just a single blue mana if youre willing to exile blue cards from your hand