Upon defeating both the Demon in Pain and the Demon from Below in the first phase, the true boss, the Demon Prince, emerges. This second phase is a significant escalation in difficulty, as you now face a single, immensely powerful entity. The Demon Prince retains many of the attacks from its constituent parts but amplifies them with increased speed, range, and damage. Its primary offensive capabilities include devastating fire-based attacks, such as a sweeping flame breath that covers a wide area, and powerful melee combos with its massive claws.
A critical mechanic in this phase is the Demon Prince's ability to heal. If you allow the Demon Prince to remain idle for too long without engaging it, it will begin to regenerate its health. This emphasizes the need for consistent aggression. However, this aggression must be tempered with careful observation and precise dodging. The Demon Prince's attacks are heavily telegraphed, but their execution is swift. Learning the timing of its lunges, slams, and fire breath is essential for survival. Weapons that deal high burst damage or have good stagger potential can be very effective here, allowing you to interrupt its attacks or create openings for critical hits.
The Demon Prince is particularly weak to lightning damage and is also susceptible to frostbite. Weapons or spells that exploit these weaknesses can significantly shorten the fight. If you are struggling with melee, consider using ranged sorceries or pyromancies, provided you can maintain a safe distance and find openings to cast. The arena itself offers some limited cover, but the Demon Prince's AoE attacks can quickly render it ineffective. Perseverance is key; this is a test of endurance and mastery of Dark Souls III's combat mechanics. Do not be discouraged by repeated attempts; each failure is a learning opportunity.