Hizell was a very powerful dragon and involved in the Dragon-Cubi War, murdering Siar, most of her clan, all of Fa'Lina's clan, and other unnamed victims. We know he is Pyroduck's father since Pyroduck was given to Fa'Lina by the Dimanika Clan because his father had been the dragon who killed her clan. Little else is known about him at this point in time, but it seems unlikely that he is the dragon Dan owed money to in the Warrior for Hire DMFA arc (i.e. simpleminded as opposed to murderous).

Reference links:

Aniz mentions that Fa'Lina's clan was exterminated: Abel's Story, Part Two, #15

Hizell is referred to in flashback: Abel's Story, Part 2, #54

Pyroduck meets his father: DMFA #1072

Fa'Lina confirms that Hizell destroyed her clan. DMFA #1586: Examples.