Deathwatch Shadowbreaker, What was Epsilon's plan? Why would it doom humanity?

I've just rinished reading Deathwatch: Shadowbreaker and I still don't understand what the experiments and operation blackseed was all for. She tbought she was saving the imperium from the tyranids somehow? Something about the warp around the Tau planet she wanted to go to? Why were the Eldar so adamant to stop her and learn her secrets?