D&D 5E Sentinel for a rogue with a whip, a shortsword and booming blade and sentinel?

[DND_Reborn]

Sentinel only allows a melee attack. It does not say that the attack you get as a reaction for being 5 feet within someone who gets attacked is an oa. So you would not be able to use booming blade with the extra attack that sentinel gives you. Now if they disengage from you then yes you could use war caster to cast booming blade as they move away from you.

True. I thought Sentinel said you can use your reaction to make an OA. Thanks for the correction.

 

[Charlaquin]

Bugbears with whips have a reach of 15', that has to be ripe for something neat.

 

[Hawk Diesel]

I feel like you would need a few things to make this function in the way I think you are intending. First, you would need warcaster if you wanted to use booming blade as a reaction. You would also need spell sniper to use reach with booming blade. This is because booming blade is explicit in having a range of 5' and spell sniper would double that to 10' which would allow it to function when attacking a creature 10' away with a whip. You could still hit booming blade with a whip, but only it if the target is 5' away.

As to Sentinel specifying an attack when activated rather than an opportunity attack in the feat's third bullet point, and warcaster specifying that it only triggers on opportunity attacks made when enemies move, I'd allow it to work. That's a lot on investment for a gimmick.