If using glyph magic does drain King, then it likely wouldn't be as big of an issue when he gets bigger. His magic reservoir would likely increase to such a huge amount that Luz could activate dozens of glyphs without a single yawn from King.
If using glyph magic does drain King, then it likely wouldn't be as big of an issue when he gets bigger. His magic reservoir would likely increase to such a huge amount that Luz could activate dozens of glyphs without a single yawn from King.