I'm playing a warlock in 5e. I've been peeking through Google, but have found contradicting answers. What IS the Expanded Spell List? Is it a second spell list? Is it just spells I can get just like any other spell? What is it?
Each Patron has a list of spells. In the description before the spell list, it says:
The [patron] lets you choose from an expanded list of spells when you learn a warlock spell. The following spells are added to the warlock spell list for you.
So the expanded spell list is not a list of auto-obtained spells (like cleric domain spells), but rather spells not normally on the warlock spell list that you are allowed to take as one of your normal warlock spells in addition to the standard list on pg 210 of the PHB.
For example: as a 3rd level Fiend Warlock, you know 4 spells, and you can choose from any of the combined choices of warlock spells on pg 210 of the PHB and the spells on the fiend expanded spells list (p. 109), but the Warlock will still only know 4 spells.
The Expanded Spell List feature basically just says this:
The following spells are added to the warlock spell list for you.
So what does that mean? It means that wherever something references the Warlock spell list, it means all the spells normally on the Warlock spell list as well as all the spells on the Expanded Spell List.
Where this is usually going to matter is learning new spells. So, for example:
At 1st level, you know two 1st‑level spells of your choice from the warlock spell list.
So, when it says you know 2 1st-level spells of your choice from the warlock spell list, it actually means you know 2 1st-level spells of your choice, each of which can come from the list of spells in the back of the book under the Warlock heading, or the list under your Expanded Spell List feature.