Defense Grid
Artifact
Each spell costs 
more to cast except during its controller's turn.
It lights up the sky, beckoning enemies only to repel them.
Illustrated by
Ninth Edition#293RareEnglishLegality
Rulings
Oct 4, 2004
The additional cost can be reduced by effects that reduce costs, such as Sapphire Medallion.
May 1, 2006
In Two-Headed Giant, spells don’t cost extra on your own turn.