Why do Warlocks only have spells up to 5th level? What's the correct progression for their slots?
Instead of a “Spell Slots for Spell Level” table, the warlock just has two columns: one for number of slots and one for slot level, because all of their slots are the same level.
However, the table on PHB p. 106 only goes up to the 5th slot level. It looks like the higher levels are misprinted as they should go up to 9th level slots (as the warlock's spell list has spells of 6th, 7th, 8th, and 9th levels). What is the correct progression? Is a warlock’s slot level the same as the maximum slot level of a wizard of the same class level?
The book is correct. For the reason why level 6-9 spells exist for the Warlock when their Pact Magic slots cap at 5, check the Mystic Arcanum class feature on PHB p.108.
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.
It's simulating the once-a-day spell slots that other casters get (presumably, they didn't want you recasting a 9th level spell every time you rested throughout the day), but there are a few minor differences.
- It's not technically a spell slot for anything that cares specifically about spell slots (such as the feature below that in the PHB, Eldritch Master), as mentioned.
- Unlike most spellcasters, you only ever "know" (as an arcanum) one spell each of 6th-9th level.
- You cannot 'upgrade' spells to those 6th-9th level 'slots', as they aren't really slots. You always cast the 6th level arcanum at 6th level, the 7th level arcanum as 7th level, etc.
Point 3 might do well to give an example of what you can't do, e.g. list a low-to-mid-level spell that a wizard could cast from an 8th-level spell slot for greater effect and point out that the Warlock can, at best, cast that spell using a 5th-level slot (with the corresponding numeric values for the spell chosen).