There are a lot of variations to this but I will give the solution that works in the puzzle and I'm pretty sure that in the branches that this works too.
|| The idea is that we are going to use Tainted Pact to find Rain of Filth to generate an extra 4 black mana and use the blue ritual to get the treasures we need.
1. Tap the 2 dual lands to cast Tainted Pact. Hold priority and cast Pact of Negation and hold priority again and tap one of the lands to cast Offer targeting Pact, netting 2 treasures.
2. Use Tainted Pact to find Rain of Filth.
3. Cast it with the last remaining land and then sac all lands to have 4B available. We now have 2 Treasures and 4B available.
4. Crack a treasure for U and use 2B to cast Valley Floodcaller.
5. Cast a Lotus Petal.
6. Use the remaining 2B to cast D Tutor, finding Underworld Breach.
7. Crack the last treasure and the petal to cast Breach.
8. Escape Lotus petal twice to generate UU.
9. Cast Brain Freeze. The storm count is now high enough to repeat this, mill ourselves until we find LED . Eventually we find Borne and we can cast that and then escape Thassa for the win.||
