Constellar Siat (HA07-EN041) [Hidden Arsenal 7: Knight of Stars]

If your opponent controls a monster and you control no monsters, you can Special Summon this card (from your hand). Once per turn: You can target 1 "Constellar" monster you control or in your Graveyard; this card's Level becomes the current Level of that monster. This card cannot be used as an Xyz Material, except for the Xyz Summon of a "Constellar" monster.