add new monsters, spells, and rules

src/pages/spells/improved-invisibility.md

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+---
+title: Improved Invisibility
+description: The Improved Invisibility spell for OSR gaming.
+date_pub: 2023-10-27T22:22:24.000-04:00
+section: spells
+content_type: feature
+short_code: s11
+tags: arcane
+---
+
+<div class='headlessTableWrapper'>
+
+|              |                       |
+| ------------ | --------------------- |
+| **Level**    | 4                     |
+| **Duration** | 4 rounds +1 per level |
+| **Range**    | Self or touch         |
+
+</div>
+
+Caster or creature touched becomes invisible.
+
+-   Any clothing or gear carried by a target creature is also rendered invisible while carried.
+-   Target can attack while invisible.
+-   Casting a spell while invisible breaks invisibility.
+-   Invisible creatures get +4 to AC and saving throws against opponents who are aware of them, but can't see them.
+-   Invisible light sources still cast light.