Board Thread:Wiki discussions and announcements/@comment-26053782-20150129202217/@comment-26053782-20150622104132

Hmmm, a bit back a few things changed:

"Cassiopeia gets less AP when combined with %AP amplifying effects

Last patch we modified how the first Dragon buff interacts with Rabadon's Deathcap (from multiplicative to additive, with an expected ~1-2% ability power change), but failed to document the larger hit to Cassiopeia, who was tripling (quadrupling?) down on it with her passive, Dragon buff, masteries, and Rabadon's Deathcap. We're documenting it now and realize this is late game Cassi nerf, but we're watching how this shakes out first.

LATE GAME CASSI NERF

Dragon buff and Rabadon's Deathcap now interact additively rather than multiplicatively. When these two ability power % modifications (and masteries) interact with Cassiopeia passive, they now give roughly ~12% less bonus ability power at late, late game."