D&D 5E Attacking a creature while in it's belly?

[Rya.Reisender]

As the creature doesn't see you, you automatically get advantage at least from that.

And I'd argue that hitting a creature while you're inside its belly is easier than normal, so normal DM-granted advantage is likely too.

 

[jasper]

The dice just hate your pc today. It takes a hit (bite), a failed grapple check, another hit (bite) to be swallowed. I stick with restrained and blinded. So it takes two rounds before you get your weapons acid washed. While inside the sum affect is disadvantage on your attacks. You are not going to be hit because you are inside, so the advantage to attacks are gone.