You've assaulted the Revenant Fortress once and defeated Fikrul's minions, but to little effect. He was able to bring them back to life using the paracausal power of his Echo. Eido has previously mentioned a captured House Salvation apothecary trapped in the Fortress who might be able to help with tonic research—Ixis and Eido together could turn the tide. Eido is attempting to reach you on radio.