The area covered by Hell Spikes increases at higher levels, becoming much more effective on level 4 or 5. The spell does physical damage, so can not be resisted. Since it does not require a target, you can use a shadow potion or Fade From Sight, cast Hell Spikes near an opponent to do damage without turning visible. Alternately, Blind + Hell Spikes is an effective combination.
Save the game, boost the spell as much as you can and try it out.
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