I found something interesing.
Grave Cleric can give enemy vunerability to next atack let's calculate:
Assassin rogue with 20dex, sharpshooter who deal crit on suprised and with second feat he can double damage so he deal all dmg as x4.
Thanks to vunerability he can deal x8
8d8 (8x bow) + 20 (4x dex) + 40 (4x sharpshooter) + 80d6 (8x sneak)
Average is about 377 damage.
I think that as DM i would be so much suprised with one shot one kill for example Balor ^^

What feat lets someone deal double damage, so the assassin gets quadruple damage?

My math is a little different:
It's a single attack, might as well use a heavy crossbow if you can:

2d10 for Heavy Crossbow for critting then doubled with Vulnerability
6-10 damage: that's +3 to +5 damage for 16 to 20 Dex doubled with Vulnerability
20 damage: that's +10 damage from Sharpshooter doubled with Vulnerability
4d6-20d6: that's 2d6-10d6 Sneak Attack damage for critting from a 3rd to 19th lvl Assassin,  then doubled with Vulnerability

A Grave Domain Cleric can do this as early as lvl 2, it just takes an Action.
So minimum lvl 3 Rogue to be an Assassin.

I did lower some of the possible numbers because while it's "cool" that a lvl 20 Assassin can do that kind of thing you expect it from a lvl 20 Assassin.  The interesting thing about this combination is how early you can start using it.  It's an effective Alpha Strike from level 3 on, especially if you have a caster who is willing to blow an Invisibility on the Assassin before a fight.

I believe the rules for doubling an already doubled number is to triple the original...

if you had 6 damage for instance, you wouldnt quadruple it to 24, but triple it to 18.  The next step is coming up with a way that a grave cleric would be wandering around with your hero's, i'll admit I havent read it entirely but doesnt sound very groupy to me

Let me just clarify a few things. 

• Multiplication is commutative.  This means that it doesn't matter what order you apply the various doublings in. 
• Whichever way you write it, two times two equals four. 
• There is no rule in D&D 5th Edition that overrides the above equation.  2 times 2 still equals 4. 
• This combo requires that the enemy be surprised, that the Assassin go before the enemy (because the enemy will cease to be surprised on its turn), that the Grave cleric go before the enemy (not necessarily before the Assassin, thanks to the Ready action), and that the enemy fail a Con save.  And, of course, that the Assassin hit the enemy. 
• If the Assassin is 19th or 20th level, has 20 Dex, and uses Sharpshooter and a +3 heavy crossbow, the attack will be made at +9 to hit (with advantage) and deal 44 (8d10) + 280 (80d6) + 20 + 40 + 12 damage, or an average of 396. 
• The features involved are the Assassin's 3rd-level feature Assassinate, its 17th-level feature Death Strike, the Grave cleric's 2nd-level Channel Divinity: Path to the Grave, the rogue's Sneak Attack, and, last and definately least, the Sharpshooter feat. 
• Not counting the ancient blue dragon Iymrith or creatures with resistance or immunity to magical piercing damage, there are 13 monsters in the game that can survive that.  They range from CR 20 (Ogrémoch) to 30 (the tarrasque and Tiamat).  The full list, sorted by CR, is here. 

Inflict wounds would work very well as well, especially at lower levels