The verb 'to enthral' is spelled thus, with one '-l-', in British English. (In American English, it has two: enthrall; and dictionaries of American English record an archaic variant spelling, inthrall.) The inflections of the verb have two in both varieties: past forms enthralled and present participle enthralling.
The associated noun enthralment has only one '-l-' in both varieties.
- Etymological note: the verb 'to enthral' is derived from the Old English noun thrall, which means in different contexts 'slave', 'serf'; or 'slavery', 'servitude', 'bondage'. So someone who is enthralled by something (or someone else) is literally 'in bondage [to]', 'enslaved [by]', or 'captivated' that thing or person, and an enthralling book, story or film is one that 'captures one's interest, attention or affection, etc'.
You may also want to see a note on the spelling pattern involved at -l - -ll-.