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.
Mystic Arcanum
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
- Eyebite
- 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.
What are the ways in which the 6th-level mass suggestion spell is better or worse than the 2nd-level suggestion spell?
Because Mass Suggestion hits up to 12 targets.
That makes its usability, well, 12 times better than regular Suggestion. Consider a fight against a group of enemies. Mass Suggestion can target possibly all (or at least a large portion) of them. You could turn a villain’s thugs against himself. Suggestion could not do it.
Because it requires no concentration.
You can take damage at will without losing the spell’s effect. You can cast other concentration spells while Mass Suggestion is working. This also implies that you can have several Mass Suggestion spells active at once, as long as you have a long rest between one act of casting it and the next.
Longer range and duration.
It also has double the range and lasts 24 times longer (24 hours is a really long time).
Level
The level 2 Suggestion automatically fails against a Dispel or Counterspell, but the level 6 Mass Suggestion forces a DC 16 (10 + spell level) save to negate it (props to Joel Harmon for that).
However
Mass Suggestion is indeed more expensive than Suggestion. For Warlocks, you can only cast Mass Suggestion once per long rest, while you can cast several Suggestion per short rest.