You are deathly afraid of being enclosed. If you are underground or in an enclosed area that measures less than 10ft on two or more sides, you suffer disadvantage on all attack rolls, ability checks and saving throws.
FLAW. This is a flaw feat. At your DM's discretion, you may take this feat at level 1. If you do, you may also gain a feat, or increase two of ability scores by +1 (or one by +2).