I want to suggest 2 type of tiles for you because they are very special and well deserve clarification and application.
First one: Bubble blocks.
You can buy it from the Party Girl in Hardmode.
It is very awesome because every tile can only be solid or non-solid, and it can't be both, - well, except for the Bubble blocks.
It is not solid when dealing with players, NPCs, and dropped items, but is solid when dealing with tiles, wirings and liquids in the world. It may have more than its original use: containing liquids.
About its original use it is easy to think of using bubble to create floating lava, making lava both more effective and more easy to use as it won't affect spawning because it is taken as non-solid block when spawning NPCs, so NPCs just spawn on the solid blocks BELOW the bubble blocks and the game only tests whether there is liquid ON the spawning tile, not more higher.
You can also use more layers of lava, even to fill your farm with bubbles and lava in a staggered pattern. They are the cheapest traps that provides free damage. (Because lava's immunity counter is independent from other timers. Although it may be not the best traps as the immunity time is 0.5 second and the damage is merely 50 with some even immune to lava.)
And they provide free bonus: they clear nearly all the coins, avoiding despawning of critical items. (Is that an advantage or disadvantage?)
Other usages example: As testing whether an NPC house is valid requires tile information, it is processed in the latter section, 'tiles and liquids', in which the bubble blocks are solid, so bubble blocks are valid building material - suggesting that they can be used to build a valid house with neither friendly NPCs nor hostile mobs taking it as a barrier between inside and outside.
As this thread wants NPC to contribute some DPS, this may be a good option when you have thought of how to keep the NPCs safe. (Well, you can make the shooting hole bubble.)
(Well, I can't place statues on bubble blocks because placing tiles is processed in player using items where the bubble blocks are not solid. If you can circumvent this, you may float a statue on a bubble, by which you can achieve the same result as the floating glitch!)
Feel free to add more example here taking advantage of the different handling of this tile in different regions!
Second one: Target Dummies.
It is obtainable pre-hardmode, but it is an awesome object.
As far as I see in the code, when you have both horizontal distance and vertical distance of less 1600 pixels(100 tiles) from a target dummy, the dummy will be activated. (EDIT: If you are away from an Dummy NPC for 10,000 tiles in distance or you are dead, it will deactivate, although it is very unlikely) In activation it will spawn an NPC representing himself. The NPC is used to show damage and debuffs.
The NPC has 1000 hp(well, no much use, but what if you shoot a bullet that deals over 1000 damage?), 0 value(drop 0 coins when defeated, so that lucky coin won't work against a dummy), 0 damage and 0 defense, 18 * 40 in size, immortal, not friendly, and [EDIT: collide with tiles(thus hoikable)].
Well, too much useless stuff... But it is an NPC, so many things that you can use against an NPC may work on Target Dummies. You can teleport him, for example, however it will not glitch much because it just despawns its NPC and respawns when it found that its NPC has been transported. [EDIT: I made the code wrong.. Actually when a targer dummy updates it don't check the NPC's actual x-y coordinate, but only registered tile coordinate when spawning the Dummy NPC. So you can teleport them away as you like.]
My example: Cheating immunity out of dummies using unicorn mount(well, slime mount can grant immunity as well, but it seems to be harder to use.)
Another inspiration although I didn't try that: eliminating NPCs using Dummies. Spawning enough dummy NPCs may help reach the NPC's upper limit 200, so that you can eliminate NPC spawn. Well I haven't tried that, and that may be done even in 1.2.4.1 if you prepare 200 birds and release them in a small box on screen...... Warning here: it may cause bugs and/or crashes....
[One epic example by @z26 : Use target dummies as hoik actors. They are immobile, but they can be teleported and can collide with tiles, thus they can be hoiked. Moreover, they can hit pressure plates and are affected by gravity.]
Waiting for you applying more dark magic that are applicable to any NPCs to the Dummies and bringing results here!
They are great tiles as they have an unique rule in Terraria. And I believe T-MEC can figure out more creative usage for special blocks introduced in 1.3!
First one: Bubble blocks.
You can buy it from the Party Girl in Hardmode.
It is very awesome because every tile can only be solid or non-solid, and it can't be both, - well, except for the Bubble blocks.
It is not solid when dealing with players, NPCs, and dropped items, but is solid when dealing with tiles, wirings and liquids in the world. It may have more than its original use: containing liquids.
About its original use it is easy to think of using bubble to create floating lava, making lava both more effective and more easy to use as it won't affect spawning because it is taken as non-solid block when spawning NPCs, so NPCs just spawn on the solid blocks BELOW the bubble blocks and the game only tests whether there is liquid ON the spawning tile, not more higher.
You can also use more layers of lava, even to fill your farm with bubbles and lava in a staggered pattern. They are the cheapest traps that provides free damage. (Because lava's immunity counter is independent from other timers. Although it may be not the best traps as the immunity time is 0.5 second and the damage is merely 50 with some even immune to lava.)
And they provide free bonus: they clear nearly all the coins, avoiding despawning of critical items. (Is that an advantage or disadvantage?)
Other usages example: As testing whether an NPC house is valid requires tile information, it is processed in the latter section, 'tiles and liquids', in which the bubble blocks are solid, so bubble blocks are valid building material - suggesting that they can be used to build a valid house with neither friendly NPCs nor hostile mobs taking it as a barrier between inside and outside.
As this thread wants NPC to contribute some DPS, this may be a good option when you have thought of how to keep the NPCs safe. (Well, you can make the shooting hole bubble.)
(Well, I can't place statues on bubble blocks because placing tiles is processed in player using items where the bubble blocks are not solid. If you can circumvent this, you may float a statue on a bubble, by which you can achieve the same result as the floating glitch!)
Feel free to add more example here taking advantage of the different handling of this tile in different regions!
Second one: Target Dummies.
It is obtainable pre-hardmode, but it is an awesome object.
As far as I see in the code, when you have both horizontal distance and vertical distance of less 1600 pixels(100 tiles) from a target dummy, the dummy will be activated. (EDIT: If you are away from an Dummy NPC for 10,000 tiles in distance or you are dead, it will deactivate, although it is very unlikely) In activation it will spawn an NPC representing himself. The NPC is used to show damage and debuffs.
The NPC has 1000 hp(well, no much use, but what if you shoot a bullet that deals over 1000 damage?), 0 value(drop 0 coins when defeated, so that lucky coin won't work against a dummy), 0 damage and 0 defense, 18 * 40 in size, immortal, not friendly, and [EDIT: collide with tiles(thus hoikable)].
Well, too much useless stuff... But it is an NPC, so many things that you can use against an NPC may work on Target Dummies. You can teleport him, for example, however it will not glitch much because it just despawns its NPC and respawns when it found that its NPC has been transported. [EDIT: I made the code wrong.. Actually when a targer dummy updates it don't check the NPC's actual x-y coordinate, but only registered tile coordinate when spawning the Dummy NPC. So you can teleport them away as you like.]
My example: Cheating immunity out of dummies using unicorn mount(well, slime mount can grant immunity as well, but it seems to be harder to use.)
Another inspiration although I didn't try that: eliminating NPCs using Dummies. Spawning enough dummy NPCs may help reach the NPC's upper limit 200, so that you can eliminate NPC spawn. Well I haven't tried that, and that may be done even in 1.2.4.1 if you prepare 200 birds and release them in a small box on screen...... Warning here: it may cause bugs and/or crashes....
[One epic example by @z26 : Use target dummies as hoik actors. They are immobile, but they can be teleported and can collide with tiles, thus they can be hoiked. Moreover, they can hit pressure plates and are affected by gravity.]
Waiting for you applying more dark magic that are applicable to any NPCs to the Dummies and bringing results here!
They are great tiles as they have an unique rule in Terraria. And I believe T-MEC can figure out more creative usage for special blocks introduced in 1.3!
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