Spectral Arachnid

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Spectral Arachnid would look a lot like the cave spider, like this spider with being triple the size of the cave spider, except it would render similiar to phantoms like this showing only it's eyes and maybe a few stripes.

• Biome Spawn: Primarily found in Ocean, sometimes found in the caves of mesa, desert and plains underground
• They hide in blocks of near the bottom of the world in granite, diorite, and andesite that is found near the bottom of the world. They don't hide when in water.
• Attack Damage: Same as the cave spider except instead of poison it inflicts you with wither and jump boost for 0.5 seconds
• Health: 15 hearts normal, 20 for hard
• Movement Speed: As fast as the baby zombie out of water and slow as drowned in the water. They can jump at you though about 5 blocks high.
• Rideable; No
• Tameable; No
• Drops: String common, {algae 0.05% chance} or {diamond ingot 0.02% chance}
• Unlike the Cave spider, it can breathe underwater.
• If found in the ocean, it will make its web out of algae
  • Algae would spawn in the oceans near warmer temps, spreading a lot like sea grass, except acts like a web. When cut with sword, you get sea grass. When cut with shears, you get the algae.
  • Algae can be used to decorate your blocks by right clicking any block would spread on it like vines creating a mossy feel, i;e mossy logs or cobblestone or etc.
  • Algae can be used in potions to make that potion spread over time to other mobs with the potions natural time limit applied in a radius of ten & spread time limit, up to .30 sec. It would also make good potions spreadable to players. Doesn't linger or show lingering bubbles, but will over time spread to new mobs as if it was a lingering potion. You can increase it's time + spread radius of chosen potion by .30 seconds by re-brewing same potion with algae again. Limited to 3 times. 
• It will show itself when a bat is near as it is food for it in the caves.

Based on real life spider. 

Not enough scary mobs in minecraft, need a new one IMO.