"X" represents any amount of pips. In other words, the way heckhound works is that it casts an overtime on the opponent. The damage you deal is ENTIRELY dependent on how many pips you used when using the spell. When using any spell that has "X" it means it will use all your pips to cast the spell. Not all "X" spells are damage spells, for example, the life spell, Dryad, heals the targeted player based on all the pips he used up for the spell.
Heckhound is not the only spell with "X" - there are definitely more. If you have any more questions, contact me.