Helmed Horror/Rakshasa Spell Immunity

[Expected]

I'm going to play a spellcaster and want to prepare myself against these foes in particular because of their immunity to spells.

Does Booming Blade/Green Flame Blade, Shillelagh, or Eldritch Blast work against a Helmed Horror or a Rakshasa?

[Unoriginal]

I'm going to play a spellcaster and want to prepare myself against these foes in particular because of their immunity to spells.

Does Booming Blade/Green Flame Blade, Shillelagh, or Eldritch Blast work against a Helmed Horror or a Rakshasa?

No. They're immune to spells. Trying to damage them with a spell's effect doesn't work.

Well, using a spell to throw something at them could work, because the damage is not from the magic, but the Helmed Horror is resistant to that, and the Rakshasa immune is immune, unless what is thrown is a magic (or adamantine for the Horror).

[Callak_Remier]

Conjured creatures are unaffected.
Focus on buffing your allies and counterspelling his spells

[History_buff]

Helmed Horrors are only immune to three spells I thought chosen when they are created. For example they might be immune to fireball, heat metal, and lightning bolt but not immune to other spells.

They are immune to force damage,poison, and necrotic damage and have advantage on saving throws against magical effects though.

Helmed horrors don’t have blanket resistance or immunity to magic.

[Finback]

Helmed Horrors are only immune to three spells I thought chosen when they are created. For example they might be immune to fireball, heat metal, and lightning bolt but not immune to other spells.

They are immune to force damage,poison, and necrotic damage and have advantage on saving throws against magical effects though.

Helmed horrors don’t have blanket resistance or immunity to magic.

Absolutely correct on all counts. I chose them for a particular encounter for this very reason - made them immune to lightning bolt, fireball and (iirc) gaseous cloud; and them teamed them up with two shield guardians. One shield guardian at each end of a corridor - one releases a gaseous cloud, which is flammable. The other one fires off a lightning bolt, igniting it. I decided if it worked, it does a fireball effect (hence protecting the helmed horrors). Best "hall of family heirlooms" trap I've made, I think. Doubly so when you consider all the dozens of swords and knives on the walls, which the PCs expected to animate any second now..

[KorvinStarmast]

Get a monk. On your team.