Introduction:

Mojang has very few default structures that spawn randomly in the world. This includes a woodland mansion, sand temple, jungle temple, water temple, strongholds, and villages. What I feel is missing from the world is something needed to be avoided once the unknown is known, a more eerie structure, something that is smaller, seems very old, fits in pretty much anywhere on the map, and also has a small graveyard with 3 graves. Having an abandoned tower that only spawns once in a world would fit these descriptions, it would be a place that doesn't have any mages, axemen, or villagers, like all the other structures in the game. The tower still has all the owners old valuables still up in his chambers (locked up) and nothing was moved out, like the owner got out in a hurry. The tower could pose the unsettling question of "Why was it abandoned?", "Could a ghost have haunted the tower?". "Is the tower cursed?", "Did the living dead force the owners out of the tower?", and "What is in that locked room and how do I get in it?".

Basics:

The plan I have for the tower is that a new wraith-type boss like The Wither named "Ethern" would spawn in it once a night and roam the outside of the tower and inside of it. This is because the old owner became a wraith that guards the tower for eternity until it is put to rest by the player. It will then drop a key that opens the door to his chambers (not accessible in any other way) with all his valuables and his personal diary that goes through how he got to where he was. One of my idea's of how he got there is that he killed his family with stabs to the back because they were low on food and he was about to perish, and couldn't cope with the sadness so he resorted to dark magic to try to bring them back but the gods punished him and turned him into a wraith to guard the bones of his family for eternity.

Mechanics:

Since there is only one tower per map (would spawn at a reasonable distance from spawn), there could be a map to it or another one of Ethern's old diaries (both could be found in dungeon chests or sold by villagers) which has the biome the tower is in. Some of Ethern's mechanics would be that he blends in with the night very well, maybe turning to just particles when sneaking up on a player at night, his boss health bar would also not be visible until you attack him or he attacks you. An attack could be a homing spell made of black particles that does 3 or 4 hearts (can be multiple homing's), and with that during the first phase could be basic attacks toward the player with 3 or 4 hearts. At the 2nd phase, an attack called the backstab would be added where he constantly tries to quickly come behind you and do an attack that does a ton of damage, the homing and the backstab could be going at the same time, meaning you would have to be running from the homing's and avoiding him coming behind you.

Conclusion:

All in all, I believe this would be a great addition Minecraft. It would be another great reason to go out adventuring, or a reason to keep your eyes peeled at night for the tower so you can avoid Ethern.