If you have a Caskets of Souls on the table at the start of your Magic phase, you add D3 power dice to your pool. For each additional Casket, you may add an additional power dice.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
If you have a Caskets of Souls on the table at the start of your Magic phase, you add D3 power dice to your pool. For each additional Casket, you may add an additional power dice.