The voiced retroflex implosive is a type of consonantal sound that has not been confirmed to exist in any language. It has been claimed that Ngad'a, an Austronesian language spoken in Flores, contains such a sound. Sindhi has an implosive that varies between dental and retroflex articulation.
The symbol for this, ⟨ᶑ ⟩ (a dee with both the tail for retroflex and the hook-top for implosive, composable as ⟨ɗ̢⟩), is not explicitly recognized by the IPA, but is supported in the Unicode Phonetic Extensions Supplement.
Read more about Voiced Retroflex Implosive: Features