An example of using LeaderLib's skill listener to create a function that runs during each different skill state.

LeaderLibSkillListenerExample.lua

--[[
Add LeaderLib's Lua folder to VSCode workspace (not the mod) so EmmyLua will use the scripts for auto-completion.
With the way EmmyLua works, if you set up these globals in a different file, 
EmmyLua will associate their usage with LeaderLib's scripts, so you'll see types/etc pointing back to the originals.
]]

--This should ideally be called in a script loaded by both client and server.
--It will import LeaderLib's various globals into your mod's table, so you can use helpers without setting them first.
--Mods.MyModTable needs to be the name you specified in your mod's ositools settings.
Mods.LeaderLib.Import(Mods.MyModTable)
--This one will import everything that's valid, instead of just the recommended globals, so take care not to conflict with global names.
--Mods.LeaderLib.ImportUnsafe(Mods.MyModTable)

local function CleanupMoveAction(handle, target, x, y, z)
	if GetDistanceToPosition(target, x,y,z) <= Ext.Random(0.5, 1.25) then
		NRD_GameActionDestroy(handle)
		return true
	end
	return false
end

local function PlayEffectAtTarget(effect, target, targetType)
	if targetType == "string" then
		local x,y,z = GetPosition(target)
		PlayEffectAtPosition(effect, x, y, z)
	elseif targetType == "table" then
		local x,y,z = table.unpack(target)
		PlayEffectAtPosition(effect, x, y, z)
	end
end

local effects = {
	"RS3_FX_Skills_Void_Power_Attack_Impact_01",
	"RS3_FX_Skills_Void_SwapGround_Impact_Root_01",
	"RS3_FX_Skills_Void_Divine_Impact_Root_01",
}

RegisterSkillListener("Projectile_Grenadier_Grenade_Vortex", function(skill, caster, state, data)
	if state == SKILL_STATE.PREPARE then
	--[[
		Called during NRD_OnActionStateEnter. Should only be invoked once until the skill is cast or cancelled, 
		as LeaderLib ignores the constant PrepareSkill events for the character after the first.
		data is nil since there aren't any targets.
	]]
	elseif state == SKILL_STATE.USED then
		-- Called once after CharacterUsedSkill/associated events, after any targets have been gathered.
		---@see SkillEventData
	elseif state == SKILL_STATE.CAST then
		-- Called after SkillCast. May have targets.
		---@see SkillEventData#ForEach
		--ForEach runs the function on each target, which may be a position or object, 
		--defaulting to objects only unless a mode is specified (-1 for all, 0 for objects, 1 for positions).
		data:ForEach(function(target, targetType, skillData)
			PlayEffectAtTarget(Common.GetRandomTableEntry(effects), target, targetType)
		end, data.TargetMode.All)
	elseif state == SKILL_STATE.HIT then
		-- When the skill hits an object during NRD_OnHit.
		---@see HitData#Success
	elseif state == SKILL_STATE.PROJECTILEHIT then
		-- Called after the extender's ProjectileHit event.
		-- data is ProjectileHitData
		---@see ProjectileHitData
		if not StringHelpers.IsNullOrEmpty(data.Target) and IsTagged(caster, "MyMod_BonusGrenadeEffects") == 1 then
			GameHelpers.ExplodeProjectile(caster, data.Position, "Projectile_MyMod_BonusVortexDamage")
		end
		local x,y,z = table.unpack(data.Target)
		local radius = Ext.GetStat(skill).ExplodeRadius
		for _,v in pairs(Ext.GetCharactersAroundPosition(x, y, z, radius)) do
			if CharacterIsEnemy(caster, v) == 1 then
				local handle = NRD_CreateGameObjectMove(v, x, y, z, "", caster)
				local timerName = "Timers_MyMod_VortexPull_"..caster..v
				local attempts = 0
				local startTimerFunction
				--Used by the oneshot timer to clean up the move action when the target is within range of the projectile target,
				local cleanupMoveAction = function()
					attempts = attempts + 1
					--Safeguard in case we're stuck in an endless loop for some reason
					if attempts >= 7 then
						return
					end
					if not CleanupMoveAction(handle, v, x, y, z) then
						startTimerFunction(250)
					else
						-- Safeguard to make sure the target is still on the AI grid
						local tx,ty,tz = GetPosition(v)
						if not GameHelpers.Grid.IsValidPosition(tx, tz) then
							TeleportToRandomPosition(v, 1.0, "")
						end
					end
				end
			--We're making a separate function for calling the timer so we can call it from cleanupMoveAction, 
			--until the target is within range or the limit is exceeded.
				startTimerFunction = function(delay)
					StartOneshotTimer(timerName, delay, cleanupMoveAction)
				end
				startTimerFunction(1000)
			end
		end
	end
end)