USER SUMMARY:
Lifthransir's Bane was the expansion, or second half, to the shareware game Castle of the Winds. If you registered Castle of the Winds, you were then able to play Lifthransir's Bane, which consisted of a much larger dungeon, with new monsters, higher levels, more souped-up items, etc.
The extra half-game didn't really add anything conceptually new to CotW, it just offered more of the same with new monsters like surly frost and flame giants, and giant enchanted bastard swords with across the board stat boosts.