The Warlock has limited high level spell casting capability due to the Mystic Arcanum feature. The Warlock can choose 1, and cannot change, spell of level 6 to cast once per long rest.
At 11th level, your patron bestows upon you a magical secret called an arcanum. Choose one 6th-level spell from the warlock spell list as this arcanum. You can cast your arcanum spell once without expending a spell slot. You must finish a long rest before you can do so again.
At higher levels, you gain more warlock spells of your choice that can be cast in this way: one 7th-level spell at 13th level, one 8th-level spell at 15th level, and one 9th-level spell at 17th level.
You regain all uses of your Mystic Arcanum when you finish a long rest.
(PHB, pg.108 / SRD, pg.48)
The spell choices for level 6 are:
- Arcane Gate
- Circle of Death
- Conjure Fey
- Create Undead
- Flesh to Stone
- Mass Suggestion
- True Seeing
Since I can take only one 6th-level spell, I want to be certain that mass suggestion is a good choice before I take it instead of something else on that list. So I want to make sure it is more useful than the 2nd-level suggestion spell that works similarly.