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Edited title to include a warning - see my post further down why I don't recommend this mod any more
Better dungeons mod - warning
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I stumbled upon this mod yesterday (because someone mentioned it had horses lol)
But, it's absolutely amazing!!!!!
[1.2.5]Chocolate quest. Mods for the adventurers - Minecraft Forum
What it does, is add extra (I assume it's not replacing) things for you to explore. It calls them dungeons, but they are really more than that. They seem to have different ones in different biomes, and they can range from a small ship in the ocean, to a sandy pyramid to a large castle. They are filled with armour-wearing mobs (some human, some vanilla mobs), and some new mobs too (so far I've encountered large silverfish and HUGE silverfish that were bring ridden by other mobs)..... and treasures.
I admit, I explored on creative, when I switched to survival while onboard a ship, to see if the crew would attack me, I was slaughtered in 2 hits.... so I would strongly advise good armour and don't let them hit you!
But it's amazing. Here's a bit of a spoiler:
The pyramids I've explored have an obsidian maze that ends with a hatch, you fall down there into a chamber with a fireball throwing boss, when you defeat that, the room fills with lava and you have to quickly escape through a newly opened exit, which takes you back outside, gives you some XP and a treasure chest!
I've attached some pics.... And yes, that whole hidden ship island thing is generated in the ocean biomes!