Stars are Right (Ongoing)
During the Doom Phase, if you turn in Elder Signs for Doom points, you also immediately receive Power equal to their face value.
Perform a ritual, and then immediately cash in the Elder Signs as reimbursement. If you can get multiple Elder Signs during the Doom Phase (such as Yellow Sign, Windwalker, Tcho-Tcho, or Black Goat), you may even GAIN power as a result of Ritualing!
Black Goat: This is another good spellbook to assist a Ghroth strategy. If you have been getting your Blood Sacrifice and ritual Elder Signs each round, you can use this as an end game attack. Use the power to decimate your opponents cultist population.
Yellow Sign: Turn in a massive amount of points in the final round to ensure you have all the power you need to finish out an action phase win. You will become a target if you were not already.