Tasha's Summon Spells + Polymorph and other such effects

[Melphizard]

Hey there folk just wanted know in case this hasn't been asked but what would your ruling be in regards to the CR of Tasha's summoning spells. The summon spells from TCoE, such as summon elemental, create a creature of the chosen type, and their power is altered by the spell's level.

Question: If my Druid friend casts polymorph on my 4th level Summon Elemental, what CR of dinosaur could he turn it into. Would it be based off my level as I casted it, the spell's level, or maybe a different factor?

Thank you!

[Sherlockpwns]

As far as I know there's no definitive answer here, but I would base it on the hitdice starting at the spell level and increasing 1 HD per spell level. So a level 3 summon would be a CR3 max, whereas a level 4 would be CR4 max, etc.

And yes, this makes it pretty useless but... the creature you summon shouldn't be the same "CR" as a player character, so it makes sense to me that it would be significantly lower. E.g. a level 7 player could polymorph into a CR 7 beast, but at best its summon is only CR4.

[JackPhoenix]

Tasha's summons (and few others from other books) don't have listed CR, so they technically can't change into anything with any CR requirements. The GM may calculate the CR using DMG guidelines, but that's up to him.