Add "invisible" mod.

mods/admin/invisible/depends.txt

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+3d_armor

mods/admin/invisible/init.lua

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+-- Code by UjEdwin
+
+minetest.register_alias("i", "invisible:tool")
+
+invisible={time=0,armor=minetest.get_modpath("3d_armor")}
+minetest.register_privilege("invisible", {
+	description = "Be invisible",
+	give_to_singleplayer= false,
+})
+
+invisible.toogle=function(user,sneak)
+	local name=user:get_player_name()
+	if minetest.check_player_privs(user:get_player_name(), {invisible=true}) then
+		if not invisible[name] then
+			user:set_nametag_attributes({color = {a = 0, r = 255, g = 255, b = 255}})
+			invisible[name]={}
+			invisible[name].tool=sneak
+			invisible[name].collisionbox=user:get_properties().collisionbox
+			invisible[name].visual_size=user:get_properties().visual_size
+			invisible[name].textures=user:get_properties().textures
+			user:set_properties({
+				visual = "mesh",
+				textures={"invisible_skin.png"},
+				visual_size = {x=0, y=0},
+				collisionbox = {0,0,0, 0,0,0},
+			})
+			minetest.chat_send_player(name, "invisible on")			
+		else
+			user:set_nametag_attributes({color = {a = 255, r = 255, g = 255, b = 255}})
+			user:set_properties({
+				visual = "mesh",
+				textures=invisible[name].textures,
+				visual_size = invisible[name].visual_size,
+				collisionbox=invisible[name].collisionbox
+			})
+			invisible[name]=nil
+
+			if invisible.armor then
+				armor:set_player_armor(user)
+				armor:update_inventory(user)
+			end
+
+
+			minetest.chat_send_player(name, "invisible off")
+		end
+	else
+	
+		if not invisible[name] then
+			invisible[name]={}
+			user:set_nametag_attributes({color = {a = 0, r = 255, g = 255, b = 255}})
+		else
+			user:set_nametag_attributes({color = {a = 255, r = 255, g = 255, b = 255}})
+			invisible[name]=nil
+		end
+	end
+end
+
+minetest.register_tool("invisible:tool", {
+	description = "invisible",
+	inventory_image = "default_stick.png",
+	groups = {not_in_creative_inventory=1},
+	on_use = function(itemstack, user, pointed_thing)
+		if minetest.check_player_privs(user:get_player_name(), {invisible=true}) then
+				invisible.toogle(user,true)
+		else
+			itemstack:replace(nil)
+		end
+		return itemstack
+	end
+})
+
+minetest.register_globalstep(function(dtime)
+	invisible.time=invisible.time+dtime
+	if invisible.time<0.5 then return end
+	invisible.time=0
+	for _, player in pairs(minetest.get_connected_players()) do
+		local name=player:get_player_name()
+		local sneak=player:get_player_control().sneak
+		if (sneak and not invisible[name]) or (sneak==false and invisible[name] and not invisible[name].tool) then
+			invisible.toogle(player)
+		end
+	end
+end)

mods/admin/invisible/readme.txt

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+Licenses: code LGPL 2.1 media CC BY-SA 3.0
+Name: Invisible
+Created by: UjEdwin
+Date: 2016-04-09
+Version: 2
+
+Hides players nametag when they are sneaking, or invisible if you have the invisible privilege.
+
+Give yourself the invisible stick, to toggle your invisible on/off
+
+/giveme i