1. The tree’s destruction broke the shield around Silvanesti, which allowed the elves to live without the fear of their life slowly being drained from them.
2. The tree’s destruction broke the shield around Silvanesti, and thus allowed the elves to live without the fear of their life slowly being drained from them.
Top answer
The destruction of the tree broke the shield of Silvanesti, so the elves could live without fear of their life draining away.
— Mister Micawber
The destruction of the tree broke the shield of Silvanesti, so the elves could live without fear of their life draining away.
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The second is much better. In the first sentence, it is unclear whether the shield itself allowed the elves to live without fear, or the destruction of the shield.