How to make a Command that changes the face?

okay so i'm really bad at scripting and i would like some help with this. I need help making a script that makes it so when i say a certain word it adds a Decal to the head on the player.

Here's what i got:

local player = game.Players.LocalPlayer
local char = player.Character
local marks = game.Lighting.mark:Clone()


player.Chatted:Connect(function(Msg)
 local msg = Msg:lower()
 if string.sub(msg, 1, 5) == "demon" then
 wait(0.1)
  marks.Parent = char.Head
 end
 if string.sub(msg, 1, 3) == "off" then
 wait(0.1)
  marks:Destroy()
 end
end)
function OnDeath()
wait()
marks:Destroy()
end

player.Chatter:Connect(onChatted)

really bad but its only what i got