One way to introduce a warning or threat in Spanish is by starting a sentence with que.
Have a look and listen to the following examples expressing a warning:
Have a look and listen to the following examples expressing a threat:
Look at the following example to understand how the word que works in this sort of sentence, expressing a warning or threat about something that could happen in the (near) future:
¡Que te dejo solo!
I'll leave you alone!
We could easily have said the same using a future tense:
¡Te dejaré / Te voy a dejar solo!
I'll leave you / I am going to leave you alone!
Note that the preposition a can go in front of the conjunction que in order to reinforce the warning or threat:
¡A que me voy!
I really will leave!
See also Conjunción.