I have a painful dense mass near my mandible area (post-auricular area). it's not hard as bone but it's firm and tender. what could this be?

Most likely it's an: Enlarged lymph node. Possibly in response to a mild infection on your head (skin, throat, nose, etc). Prob nothing to worry about as long as 1) you don't have other symptoms like fever, sweats, wt loss & 2) it's less than 1cm in size and doesn't get larger or persist. Pressing on it may keep it enlarged and sore, so try to avoid. See your doc if other symptoms, increased size/pain, or concerned.

LN vs EIC: Lymph nodes and sebaceous cysts (eic) may be commonly found in the mastoid area (near the mandible / post auricular).

Lymph node: Most likely cause reactive lymph node. Can be other things too. See ENT doc for assessment/ recommendations.