My favorite spell, good to see this <3

Always laugh with this spell.  We've used this spell creatively from shooting Glarg in the balls to combining it with an attack on an orc.

This spell is too funny

this spell isn't available in the basic rules, but rather in the player's handbook at page 237

So with agonizing blast invocation at 17th level and 20 CHA (+5)...

4 different beams of force damage attack, with 6-15 damage? Yowza.

And no saving throws for adding repelling blast/grasp of hadar invocation push/pull effects.

If you wanna go the extra mile (and maybe infuriate your DM), multiclass sorcerer and use metamagic to twin/quicken the cantrip to send 8 beams and become an Eldritch Blasting machine-gun

Haha. 48-120 (84) damage for the cost of two sorcery points. Love it. Sorcerer/Bard/Warlock builds are my favorite, too.

So, if a warlock has Advantage and uses it to cast Eldritch Blast at a level where it has multiple beams, do the individual attack rolls for each beam get to be rolled twice or just one of them?

It depends on the type of advantage. Some spells or class features grant advantage only on the next attack roll (for instance, the Fighter Battlemaster's Distracting Strike battle manoeuvre), so in that case you would roll twice only for the first blast. Other spells (and possibly class features, I don’t recall right now) grant advantage for an entire turn or a period of time (like Faerie Fire), so in that case you will be able to roll twice for all of your blasts.

You can't twin spells that are capable of targeting more than 1 target.

Twin/quicken for levels 1-4, then quicken only from 5+. Quicken fights for Bonus Action with other warlock spells and abilities like Hex/Hexblade’s Curse, but is very powerful if you have either up.

Fun fact: if you take both the Spell Sniper feat, and the warlock invocation Eldritch Spear, this cantrip can have a total range of 600 feet.

1200ft if you use the sorcerer's meta magic

I don’t believe you are able to twin eldritch blast

(As I mentioned in the first comment, twinned only for levels 1-4!)

I've been looking for this spell everywhere in wizards, then I finally see it's only for warlocks now ;-;

Magic Initiate feat can give anyone eldritch blast.

It's better then firebolt because Force is less often resisted then fire. Also making separate attacks for each beam makes it more reliable damage. However the best part of Eldritch Blast is the innovations that add extra damage and forced movement to the cantrip and those picking up via magic initiate miss out on that. 

Quote from Gay_Dog >>

Fun fact: if you take both the Spell Sniper feat, and the warlock invocation Eldritch Spear, this cantrip can have a total range of 600 feet.

 That's Exactly what I did. I died later though.

Are you even able to see that far away? Or even 600 feet away? How would that work in combat.