Emperor's Dais

When do I pay 1 energy and return a unit for Emperor's Dais?

During finalization, you decide whether to use the ability^[383.3.a] and, if you do, pay 1 and return the unit.^{[383.3.b][383.3.b.1]} Both actions form one inseparable base cost, so you must complete them together for the ability to finalize.^[740.4.a] The pending ability is removed without finalizing if no legal unit can be chosen^[402.4] or if you decline the complete cost.^[404.2] While paying the energy, you may activate an eligible Add ability with Reaction to provide it.^[444.2.c]

The unit you return is part of the cost rather than a target, and it returns to its owner's hand during finalization.^{[355.10.c][355.10.c.1]} If you have paid the cost, the ability finalizes and opponents may then react before it resolves.^{[406.2][406.4]}

What happens if Emperor's Dais leaves before its ability resolves?

A battlefield token that replaces Emperor's Dais still supplies the "here" location for its Sand Soldier, but if Emperor's Dais leaves without replacement, the Sand Soldier cannot be played because "here" no longer identifies a location. The replacing token is treated as the same game object as Emperor's Dais.^[438.1.a] Without a replacement, Emperor's Dais has no location when "here" is checked as the instruction executes.^{[359.3.e.12][359.3.f.1][359.3.f.2]}