Level 2 fighter/ level 4 gloom stalker. On the first turn of combat intake my attack and hit then use dread ambusher for the +1d8 extra attack.  Take my second attack and use my bonus action to attack. ( that gave me 4 attacks)now I use action surge and hit on my first attack do I get to do dread ambusher again? For the +1d8 attack?

Action Surge gives you another Attack action so yes.  However, you don't get extra attack until you hit either 5th level ranger or fighter so you don't get 4 attacks.

Where did your bonus action attack come from? Dual wield?

Yes I ment lv5 ranger thank you for the clarification. 

Quote from rakassan101 >>

Yes I ment lv5 ranger thank you for the clarification. 

 Ok, all good then.

I got a question, For the Dread Ambusher, is it just 1d8 or is it weapon damage + modifier+ 1d8?

I think it just does 1d8 damage of the weapons type.

But theoretically it would trigger Hunter’s Mark again? I mean, otherwise the Hunter or Monster Slayer features would be objectively better not requiring an additional attack roll. And what about bonuses granted by magic weapons such as a Longbow, +2? 

You have a +2 bonus to attack and damage rolls made with this magic weapon.

Would this 1d8 count as a “damage roll made by this magic weapon” because it is triggered by an attack made with the weapon, or is it a damage roll made by the subclass feature so therefore non-applicable?

Dread Ambusher

At 3rd level, you master the art of the ambush. You can give yourself a bonus to your initiative rolls equal to your Wisdom modifier.

At the start of your first turn of each combat, your walking speed increases by 10 feet, which lasts until the end of that turn. If you take the Attack action on that turn, you can make one additional weapon attack as part of that action. If that attack hits, the target takes an extra 1d8 damage of the weapon’s damage type.

Re-reading the ability it looks like it is a full on extra weapon attack that also deals an additional 1d8 damage.

Dread Ambusher allows you to add your wisdom mod to initiative, extra 10 feet of movement, and an additional attack.

With an extra 1d8 added to it. Holy balls that’s good!

Yeah it is a good subclass, maybe too good.

A 3-level dip on a Wood Elf Kensei. 5 Attacks/Turn with Flurry, plus Hunter’s Mark. A couple options for spell casting. Extended Darkvision and a fighting style to boot. And if it’s a Variant Ranger.... Wood Elf Magic as an option, Elven Accuracy, Mobile.... Lots of options for lots of builds. Wow.

the dread ambusher is a special attack off the 1st attack that turn.  its the weapon damage plus a bonus 1d8 of weapon type.  

so Ranger with scimitar attacks and gets hit 1) that 1d6 +4 then gets to attack again dread ambusher for 1d6 +4 +1d8

Or a double-bladed scimitar, or a longbow. Wow.

Quote from IamSposta >>

Or a double-bladed scimitar, or a longbow. Wow.

 
I'm playing a Gloom Stalker/ Assassin right now... it is so good with a Longbow, or a Rapier, or even a Dagger. 

If the monsters are surprised it's a Critical Weapon damage + Hunter's Mark Damage + Sneak attack damage +5/ +7 (Dex 20, +2 for Duelist Fighting Style)

Followed by an additional Weapon Damage + D8+ 5/7

Bugbear? They get brutal with Assassins.

It could also work well with a half-orc with Savage Attacks, to make the first attack even better if you crit. 

Yeah, but Bugbears get Surprise Attack.

Surprise Attack

If you surprise a creature and hit it with an attack on your first turn in combat, the attack deals an extra 2d6 damage to it. You can use this trait only once per combat.

Quote from IamSposta >>

Yeah, but Bugbears get Surprise Attack.

Surprise Attack

If you surprise a creature and hit it with an attack on your first turn in combat, the attack deals an extra 2d6 damage to it. You can use this trait only once per combat.

 Yes, that is a much better combination. Lets you do an extra 4d6 damage on a critical hit, instead of just one extra dice from the half-orc. Half orcs can use their ability more often, but this would be better for a stalker-character that wants to destroy the first creature they attack. 

I know, I got a Bugbear Assassin in the campaign I DM.