  One of the legacies of the {\rm Chandra} era is the discovery of AGN-inflated
X-ray cavities in virtually all cool-core clusters, with mechanical
luminosities comparable to or larger than the cluster cooling rate, suggesting
that AGN might be responsible for heating clusters. This discovery poses a new
set of questions that cannot be addressed by X-ray imaging or modeling alone:
Are AGN actually responsible for halting cooling flows? How is the AGN energy
transferred to heat? How tight is the observed balance between heating and
cooling? Using numerical simulations and a new virtual X-ray observatory tool,
we demonstrate that high-resolution, high-throughput X-ray spectroscopy can
address these questions and that the International X-ray Observatory \ixo will
have the necessary capabilities to deliver these measurements.
