What is Shield Energy?
Shield Energy is a Special Energy that was used in the Pokémon Trading Card Game (TCG). It has only been released once – in the XY Primal Clash set of 2015.
See also: What are Special Energy Cards?
The Shield Energy card provides one Metal Energy but can only be attached to a Metal type Pokémon. The card design features the Metal Energy symbol.
In the Pokémon TCG, a player can only use an Attack Move if the specified number of Energy are attached. This specified number generally includes one or more Energy that corresponds to the Pokémon type, and / or a number of Colorless Energy. Shield Energy Cards provide one Metal Energy.
Shield Energy Card Text:
The text on the card reads:
‘This card can only be attached to 
Pokémon. This card provides Energy only while this card is attached to a Pokémon.
The attacks of your opponent’s Pokémon do 10 less damage to the Pokémon this card is attached to (before applying Weakness and Resistance).
(If this card is attached to anything other than a Pokémon, discard this card.)‘
Gallery
Shield Energy Card 143/160 from XY Primal Clash
