Hello, I have a question about breath weapons. I am playing a Dragon as per approved Homebrew. I have a five foot wide by fifteen foot long line of acid. Can I target intersections instead of squares with creatures? I was thinking I could at the expense of granting all targets advantage on the Dexterity save.

Draw a line. Every square that that line goes from one side to the other (or has at least 3 feet pass through if the angles are weird) is an effected square.

Trying to make it 10 feet wide at a penalty is up to DM.

Okay, thank you. They said that would wait for any responses here before making their decision.

Technically, the way the rules work is that you are supposed to figure out the actual coverage of the AOE and any square that is over half covered (so like 50.1%) is an effected square. The rules don't want you to be able to effect 10 feet of squares with 5 feet if effect.

I'm assuming the same rules apply if I ever develop a cylinder instead of a line?

To save on time: you treat this like you would an AoE from a spell.

Info on how to rule these and what areas they effect are in the Basic Rules here: https://www.dndbeyond.com/sources/basic-rules/spellcasting#AreasofEffect