At the end of each Act, you will encounter one of the 3 random bosses for that act. The boss fight is denoted in the map by a big icon at the very top. Each boss has its own icon, giving you a chance to adapt your strategy accordingly. The preceding space on the map will always be a Camp Fire, allowing you to rest before the fight. (With the exception of Corrupt Heart, which has an elite encounter before it)
Each boss slain adds 50 points to your Score multiplied by the total number of bosses you have encountered. Slaying a boss without taking damage (not including self-damage cards such as Offering, Hemokinesis, etc.) adds an additional 50 points, and perfecting all 3 bosses adds a 200 Points bonus, overriding previous flawless bonuses.
If you have Pantograph at the start of a boss fight, it will heal you for 25 HP.
On Ascension #20 (and beyond, if added), you will fight two bosses at the end of Act 3. You cannot fight the same boss twice.
In the Mind Bloom Event in Act 3, choosing [I Am War] will allow you to fight a random Act 1 boss for a random rare relic. This boss fight does count for the Score bonuses and does activate the Pantograph relic.
After defeating the boss (with the exception of all Act 3 Bosses and the Corrupt Heart), you will be rewarded with Gold, a random Potion, a choice between 3 Rare cards, and a choice between 3 Boss Relics.