The latest Unearthed Arcana shows the following addition:

Kiss of Mephistopheles

Prerequisite: 5th level, the Fiend Patron, Eldritch Blast cantrip

You can channel the fires of Mephistopheles through your eldritch blast. When you hit a
creature with that cantrip, you can cast fireball as a bonus action using a warlock spell slot. However, the spell must be centered on a creature you hit with eldritch blast.

The new UA allows the warlock to cast Fireball as a bonus action -- essentially building in the Sorlock in an invocation without requiring a multiclass into Sorcerer. But the question is, do you actually need to know the Fireball spell (as in, choose to have Fireball take up one of your Spells Known) to be able to cast it this way?

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    \$\begingroup\$ Note to future answerers: The scenario of being a 5th level Fiend patron warlock without knowing Fireball is a possible one as (unlike similar features such as cleric domain spell lists) warlocks do not automatically know the spells in their patron's Expanded Spells list. That list is merely additional spells the warlock can learn. Noting this because there've already been 2 incorrect answers along these lines that have deleted themselves. \$\endgroup\$ – CTWind Feb 16 '17 at 0:30

No, you do not need to know Fireball - the language of "you can cast" enables you to cast it through this feature regardless of whether or not you know the spell. This Eldritch Invocation uses the same language as the ones that allow you to cast spells the Warlock usually cannot learn, such as Dreadful Word or Ascendant Step.

To supplement this answer, note that it specifically has a spell known prerequisite of only Eldritch Blast, not Eldritch Blast and Fireball. It would likely state the latter if it required that you knew Fireball in order to make use of it.

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    \$\begingroup\$ It does have the level requirement that a Spellcaster would have to have to be "powerful" enough to cast a Fireball spell, though. \$\endgroup\$ – Javelin Feb 14 '17 at 13:12
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    \$\begingroup\$ @Javelin I believe that's standard for invocations that allow you to cast a spell; I think they all require you to be of at least a warlock level where you're guaranteed to have the slot level for casting said spell. \$\endgroup\$ – CTWind Feb 14 '17 at 16:18

The prerequisite for this invocation is that you must have the Fiend Pact. This means you will learn Fireball by default at level 5 from your Fiend Expanded Spell List.

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    \$\begingroup\$ This is incorrect. Your "Expanded Spell List" does not mean you automatically know all the spells there, just that you can choose those spells which you would not have otherwise been able to know. \$\endgroup\$ – user27327 Feb 15 '17 at 16:43
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    \$\begingroup\$ @markovchain is correct; the Warlock expanded spell lists do not work the same way as, for example, the cleric's domain spell lists, which count those spells as always prepared. \$\endgroup\$ – CTWind Feb 15 '17 at 18:20

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