The spell Globe of Invulnerability is one of the most underused spells for several reasons, so much that its not worth taking the spell, or taking the greater dragonmark of the sentinel.
The lack of usefulness in my opinion is linked to several factors:
- Friendly Spells are Suppressed.
- The Globe is stationary.
- Light-Based Monk finishers are Suppressed.
- The duration is too short.
All of these factors would be eliminated if the spell was changed to a mantle effect, similar to the Demon Queen or the Black Abbot's Mantles. Of course, giving a straight immunity to all spells level 4 or lower might be too much so here's what I suggest.
- Give the spell a Duration of 1Minute/Caster Level.
- Make the spell only block so many spell levels worth of spells before it ends prematurely. I.e. 5 spell levels per caster level (100 spell levels at level 20, meaning 100 level 1 spells, or 25 level 4 spells, persistent AoE spells would reduce charges every time they would apply their effect)
- Keep the spell self only.
- Friendly effects are not suppressed, this includes inflict various wounds and harm if the user is undead.