If I have no cards in my hand, can I use Rotom VSTAR's "Conversion Star" Ability?
You must have at least 1 card in hand and 1 in your deck to use the ability. If you do, you can discard as many cards from your hand as you want.
Source: TPCi Rules Team (2023-01-19)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH125_46
SWSH125_46