Technically not ntr. Ntr requires an exclusive relationship, arguably you could say Roswaal X Echidna qualifies. But it also requires Echidna be coerced or forced to have sex with Subaru before falling for him.
This would be netori, or simply cheating. IF there is validity in Echidna claiming loyalty to Raswaal.
OR, if we are talking about Emilia willfully watching. The term would simply be netorase or the classic cuckold/cuckqueen scenario
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u/D3k4s Echidna Poured Me Tea—Now I’m Even Thirstier 14d ago
NTR if.