Reconstruction:Proto-Finnic/-sën

• *-hën
• *-sen, *-hen (front vowel harmony)

From an internal locative morpheme *-s- + epenthetic vowel + Proto-Uralic *-ŋ (lative suffix). Cognate with Proto-Samic . This etymology is incomplete. You can help Wiktionary by elaborating on the origins of this term. in the allative, *-ŋ allows for both *-n and *-k; why not here?

*-sën / *-sen

1. Illative case ending.

As per usual rules (of 'suffixal gradation'), the consonant *-s- undergoes lenition to *-h- when preceded by an unstressed syllable. Many descendants have levelled the ending to *-h-.

• Estonian: (illative case; transphonologized and exhibited by a stem change); -hV
• Finnish: -Vn, -hVn
• Ingrian: -V
• Karelian:
  • North Karelian: -h
  • South Karelian: -h
• Livonian: (illative case; transphonologized and exhibited by a stem change); -zõ, -z, -õz
• Livvi: -h
• Ludian: -h, -he, -ze
• Veps: -he, -ze
• Võro: -he, -hV (in some cases transphonologized and exhibited by a stem change)
• Votic: (illative case; transphonologized and exhibited by a stem change); -hV

Repeated illative suffix *-sëhën / *-sehen:

• Estonian: -sse
• Finnish: -seen
• Ingrian: -sse
• Votic: -sõ

Most probably derived from or of the same origin as *hän.

The possessive suffix developed by analogy with the reflexive suffix from its original form of *-sa, *-sak (see more there),^[1] after the original vowel of that suffix became obscure.

*-sën / *-sen

1. Third-person reflexive suffix.
2. (Northern Finnic) Third-person singular possessive suffix.

• As per usual rules (of 'suffixal gradation'), the consonant *-s- undergoes lenition to *-h- when preceded by an unstressed syllable.
• Also appended to the passive/impersonal infix *-tta- in at least Northern Finnic and Votic to create a passive/impersonal form.
• A plural form *-sët/*-set, *-hët/*-het with *-t probably also existed in Proto-Finnic, which would make this a third-person singular form specifically.

Reflexive:

• Finnish: (dialectal, archaic) -he
• Karelian:
  • North Karelian: -h
  • South Karelian: -h
• Livvi: -h
• Ludian: -ze
• Veps: -se, -he

Possessive:

• Finnish: -Vn
• Ingrian: -V, >? -he
• Karelian:
  • North Karelian: -h
  • South Karelian: -h
• Livvi: -h
• Veps: -ze, -nze