Warlock spell slots work differently than other classes when multiclassing. Warlocks have three spell slots per level, while other classes have two. Additionally, warlocks cannot use the concentration spell slot.
This means that when a warlock levels up, they can choose to use one of their three spell slots to learn a new spell, or to cast a higher level spell that they already know. The concentration spell slot is not usable for this purpose. .
When a warlock multiclasses with another class, the warlock’s spell slots are divided between the two classes as follows:
The warlock’s first level in the new class will grant them two of the new class’s spell slots. The remaining space is then divided evenly between the two classes, granting each class one slot per level of the warlock (3 at level 1, 2 at level 2, and 1 at level 3).
For example, if a warlock is a 3rd-level paladin and wants to multiclass as a warlock, their first level in the warlock class will give them three spell slots (two from the warlock class and one from the paladin class). Their second and subsequent levels in the warlock class will give them two more slots (for a total of 5 at levels 2 and 3), while their levels in the paladin class will give them one more slot (for a total of 4 at levels 2-4).
This system is similar to how feats work – when you gain a feat, your character gains all of its benefits regardless of what other character classes you might have. This means that if you want to multiclass as a warlock and take an archetype such as the Hexblade or Witchblade, your spells will still be based around your Warlock spells rather than those of your new archetype.
