When the Lords originally challenged the Ancient Dragons Seath, who was born without the scales that would grant him eternal life, betrayed his own kind and revealed their weakness to Lord Gwyn. Once the war was over, Seath was granted dukedom and his own archives, which he then holed himself up in. During his studies Seath developed the earliest sorceries, and performed horrendous experiments on maidens throughout Lordran. He slowly descended into madness, driven only by the desire to achieve immortality like his fallen brethren.
Despite only being present in the first game, his influence can be felt throughout the other two. Duke Tseldora as well as Najka and Tark's stories seem to have ties back to him, and King Oceiros seems to have taken a few pages out of the paledrake's book.
So the first time you encounter Seath he is technically immortal due to the Primordial crystal, so you can do all you want, but you're just going to die. After you've scraped through the Crystal Caves, he swoops in for another round. This time you're ready and he's fairly easy to beat if you keep moving.
Seath has made such a lasting impression on the series that I had to include him in the top 15.
14) Pontiff Sulyvhan
Dark Souls III
As a Sunbro, this (along with Dragonslayer Armor) is probably the boss I get summoned for the most, and for good reason. Sulyvhan doesn't mess around when it comes to fighting. His style is very elegant, and he hits hard with two greatswords.
The Pontiff takes a leaf from Darklurker's book and creates a phantom copy of himself midway through the fight, but what makes this one different is that the copy does the attack that the real Sulyvhan is about to do, allowing you to react accordingly. He is rather unrelenting in his attacks, and keeps you on your A-game for the entire fight.
In addition to an epic duel, Sulyvhan brings in some awesome lore. He is an avid supporter of Saint Aldrich, and provides the Lord of Cinder with a steady supply of sacrifices. He even managed to capture Dark Sun Gwyndolin and offered him to Aldrich as well.
Sulyvhan commands the Pontiff Knights as guardians of Irithyll, and the Outrider Knights to conquer foreign lands. Both Vordt and the Dancer are Outrider Knights, but have been transformed by the rings they are forced to wear into mindless beasts.
The Ashes of Ariandel DLC gives us a little more insight to Sulyvhan's origins, and indicate that he initially came from the Painted World itself.
13) Gwyn, Lord of Cinder
The man responsible for most of the series' events and the final boss of the original game, Gwyn is a formidable foe. This guy is absolutely relentless with his attacks, leaving you almost no time to heal from the copious amounts of damage he's likely inflicted on you already, and you're in major trouble if you can't get the dodge timing down.
Gwyn was one of the individuals who found a Lord Soul, and instigated the fight against the Everlasting Dragons. After the war, he ruled over the Age of Fire as the Lord of Sunlight, until one day when the First Flame began to fade. After several failed attempts to keep it lit, Gwyn made the ultimate sacrifice and used his own powerful soul as fuel for the Fire, becoming the first Lord of Cinder in the process.
Unfortunately, the proved to only be a temporary solution, as the fire goes through a cycle of burning out and rekindling, necessitating a new Lord of Cinder to rise up to the occasion every time. It also had the unforeseen consequence of the Undead Curse, which brought even more disparity on Lordran.
Our ultimate goal is to either take Gwyn's place as the new Lord of Cinder, or leave the world to enter the Age of Dark. I don't think it really makes a difference, honestly.
Lastly, that music! I'm going to leave a link because I like it so much. It's quite tragic, but at the same time has an air of dignity. It's as if Gwyn has resolved himself to his fate, but still wants to die on his own terms.
It's a great ending to one of the best games I've ever had the privilege of playing.