The calling

Asking if the T'au situation was ready yet, Nimrod was told it had almost matured, leaving him with a bit of time to kill. Nimrod NEEDED to free Calista; the heartache was eating at him, but at the same time, he couldn't do that without potentially losing Calista's life. As such, Nimrod knew he needed to wait a bit longer; his plans were almost ready. On both fronts.

That's when the Emperor called Nimrod. The Emperor had to move some things around, but now the time had come for the Emperor to reap what he had sown.

The Emperor wouldn't tell Nimrod what purpose he had for him, but Nimrod knew it was probably to eat his soul. Even to beings like the Emperor, Nimrod's soul was strong, pure. If the Emperor could fuse Nimrod's soul, gaining Nimrod's non-Warp-related non-physical properties, the Emperor would gain another edge. Moreover, the Emperor might be able to call lost parts of himself back, possibly even dead shards, due to Nimrod's soul's strength.

Nimrod now had a choice, try and break free or go willingly