D&D 5E Clarification on Extra Attack and Colossus Slayer

[Sotik]

So during an encounter, my ranger was going to attack a creature that was at max HP. It was asked, since she gets an "extra attack" after the first attack, would colossus slayer apply to the extra attack. As I read it, extra attack lets you take another attack when you take the Attack Action. While colossus says when a weapon attack is done, the creature takes extra damage if it is below max health.

So as a new DM I was unsure on how it would actually work. I didn't know if the extra attack would allow the extra damage of colossus slayer, or if it wouldn't count since the creatures health was max before she used the Attack Action.

 

[Horwath]

When 2nd attack happens targeted creature is not at MAX HP so colossus slayer would apply.

 

[guachi]

What Horwath said.

If the creature with max HP is hit and takes damage with the first attack, the second attack will trigger Colossus Slayer.

The Ranger in our party was so happy he hit fifth level as he no longer took the Ready action to try and use his Colossus Slayer after someone else later in the initiative count hit something.