If I turn an erinyes into a lamb using magic, can I talk to it with the talk to animals spell?

It depends entirely on which spell you used to turn the erinyes into a lamb! This is a fantastic question because it highlights a very subtle distinction in the 2024 shape-shifting rules.

If you used the 4th-level Polymorph spell: You won't be able to talk to it. The 2024 rules explicitly state that while a polymorphed target assumes the stat block of the chosen Beast, it retains its creature type D&D 5e SRD 5.2 — Spe.... An erinyes is a Fiend hailing from the Nine Hells D&D 5e SRD 5.2 — Mon...; therefore, even when it looks and acts like a fluffy lamb, its creature type remains Fiend. Because Speak with Animals only allows you to communicate with Beasts, the magic won't connect.

If you used the 9th-level True Polymorph spell: You can talk to it! Unlike standard Polymorph, the description for True Polymorph states that the target only retains its Hit Points, Hit Point Dice, alignment, and personality D&D 5e SRD 5.2 — Spe.... Because "creature type" is intentionally missing from this list, your erinyes genuinely becomes a Beast. Speak with Animals will work perfectly here! However, since the spell specifies it retains its original personality D&D 5e SRD 5.2 — Spe..., it will likely just bleat very diabolical and insulting things at you.

Note: This is just a taste of how LorePanic can quickly resolve niche rules interactions! With the full app, GMs can upload their own rulebooks, adventure PDFs, and session notes to get this exact experience for any homebrew or official campaign.