The whole deck is about getting Istanu and Doomgate on board.
Doomgate lets you kill Istanu so that another creature becomes Istanu, allowing you to carry on attacking. This is useful when Istanu is out of position or has traded a creature with <5 attack.
Biomancer helps draw Istanu. It's a prime Failed target.
Time of Legends should be held until you have Istanu so that you will draw Doomgate. Therefore other legends such as Aurora and Fugoro are omitted.
Failed Experiment lets you save Faeria on the cost of Istanu while allowing him to enter play instantly. Spare copies can be used on Biomancer, Lore Thief, or Mirror Phantasm in a pinch.
Rain of Fish provides lots of tokens for Istanu to transform into. Since you are going to 4 lakes for this it makes sense to include Frogify.
I tried a Faeries version but they are costly if you lose them, this sac version has better tokens.
Mantas are preferred to removal like Soul Drain and Spirit Theft because they provide more tokens. Rain of Fish acts like removal.
Doomgate / Mirror Phantasm / Demon Wrangler is also good for targeting a Fish which will die because it is on a non-lake land. You can also Phantasm a Fish and climb onto a land for a surprise clear or face hit.