The description of the spell states you can store a spell of 3rd level or lower by casting it as part of the Glyph of Warding.
Does this require the use of one spell slot (because you are casting it as 'part' of casting the glyph), or two spell slots (one for the glyph and one for the stored spell)?
Also if it does expend two spell slots, when is the second slot expended? Is it expended upon storing it? Or upon the glyph's activation, which then casts the stored spell?
Best Answer
Two slots -- one for the stored spell, and one for the Glyph (which must be the same level slot (or higher) as the spell being stored).
The Spell Glyph option of the Glyph of Warding spell (PHB, p. 246-247) states:
Both slots are expended when the Glyph with the stored spell is created, as that is when the spell caster is casting both spells.
There's nothing in the spell description that says it allows an extra spell to be cast and stored without using a spell slot. Using a spell slot is how casting and storing spells both normally work (per p. 201 of the PHB), so that's the general rule, and it holds unless a specific rule says otherwise for a particular application - which in this case it does not.