If it's an Action-RPG or a Dragon Age-style RPG (let's assume a 4-person party, for the sake of what I'm going to type), then the fight begins with me sitting on a massive throne as the player makes their way through a gauntlet of challenges, needing to navigate through traps and defeat enemies that are tougher than the ones found in the final dungeon of the game. Once they reach my throne room, the doors seal behind them and lock them in and I begin to monologue. I stand and slowly walk toward the center of the room (shaped like a circular arena for convenience, because I knew this day was coming), and if it's a party-based RPG then all party members except for the 'main' character either become mind-controlled or otherwise incapacitated. And so begins a lengthy, multi-phase fight.
It begins with a 1-on-1 in the center of the arena. If the player picked a mage or archer as their class, then I will engage in ranged combat accordingly, casting Reflects or Barriers that the player needs to either dodge or break in order to continue damaging me. At 90% HP, I use my first one-hit-kill attack, which is either deflected by the appropriate gathered item or absorbed by one of the immobilized party members, removing them from the fight. After this point, the other party members join in the fight against me.
After 90%, I begin using cleaving and AoE attacks in addition to the Reflects, Barriers, and normal Melee/Ranged attacks, so the player needs to position their party accordingly to minimize damage received, while maintaining constant eyes on the battlefield because one of the two AoE spells I cast will leave a patch of damaging ground where it landed, ever shrinking the size of the field they have to move around and pressuring them to push me in to the next phase quickly.
At 80%, I knock back any characters within 20 yards of myself and begin absorbing all of the corruption on the ground. This can be interrupted if a party member reaches me in time, and will greatly simplify the next phase if accomplished. If interrupted, the phase plays out exactly like 90-80%. If I manage to absorb all of the corruption, however, I gain multiple new moves, all of which use a certain amount of the corruption that I absorbed.
The first is a spell that targets the player character currently farthest away from me, and brings them down to 1 HP after three seconds. The cast is interruptable.
The second is using the corruption to empower my weapon, placing a debuff on my current target that makes them unable to be healed for five seconds.
The third is a channel that uses the corruption to generate a damage-absorbing shield around me, which is interruptable but will lead to me empowering the weapon instead if stopped.
This phase ends whenever I reach 70% health or 0 Corruption, whichever happens first.
The next phase begins with an explosion of darkness that makes the entire room go dark. Pillars form around the edges of the darkness, providing light to the player but also showing distorted images of their broken bodies, signifying their defeat. I am untargetable during the sequence, but no damage is being dealt to the party either. This is the section where, if another item was not obtained, a second party member will be removed from the fight, as the pillars will start cracking and splintering, picking one character at random whose pillar shatters completely and then they subsequently get absorbed into the darkness.
As the darkness clears away, I reappear on the throne with wings of shadow enveloping my body. I allow the party twenty seconds of free damage as I monologue once more before reengaging them in combat. I use all of the abilities from the first two phases, and in addition have two extra attacks, which are quick, low-damage swipes using the wings that are extremely difficult to avoid.
At 65%, I extend the wings to full span and fly up into the air, rendering melee characters unable to reach me unless they have proper talents or obtained another of the side-quest items, which will drag me back down to the ground and continue the phase like normal.
If I'm not dragged back down, I'll stop using melee attacks and stop casting Barriers or Reflects, and begin firing shadow balls at the ground where party members are standing. It will be telegraphed three-to-five seconds in advance so that the player has adequate time to move away from the area. The balls will leave patches of corrupted ground like in the second phase. Once I reach 60% health, I'll shout out "Enough! I tire of this game.", land, knock away any party members that are near me, and begin absorbing corruption again. However, this time is scripted, and when I finish absorbing I unleash a massive shockwave that reduces the entire party's health to 1 HP. The shockwave will occur whether I'm dragged to the ground at 65% or not.
It is at this point that a mandatory quest item comes into play, as a stone the main character holds begins emitting a light and a cut-scene is triggered. During the cut-scene, a legendary creature native to the game's mythos comes to the aid of the party, having been called by the stone. The party is fully healed and the creature is henceforth controllable by the player, adding a temporary character to their party. The creature has an arsenal of offensive and defensive magic that can be used to target me or the party, and has high health and melee damage output, meaning that it's useful for taking my attention off of the other party members.
60% to 50% plays out like the second phase again, with the patches of corruption only being temporary this time because of the power of the legendary creature helping the party. Once I reach 50%, the outside edges of the arena begin crumbling, slowly chipping and falling away, permanently reducing the size of the arena that we're fighting in. The arena continues crumbling from the outer edges in until the party manages to bring me down to 45% - At which point the walls surrounding the arena fall away and the backdrop turns to a hellish scene of agony and blistering corruption. Pieces of the walls fall on to the arena, dealing damage when they land if a character is standing beneath them and remaining on the field until they are destroyed by various means. They can be used to Line-of-Sight magical spells that I cast, and will crumble after a few hits.
Once I'm brought down to 30%, another cut-scene is triggered. During this cut-scene, I use the same corruption spell from the third phase to weaken the legendary creature enough to drive my sword through its skull, killing it. A shockwave blast sends it flying away from me, and if the player hasn't acquired the appropriate item, the corpse of the creature collides with the third party member, sending both flying into the abyss below, leaving just the 'main' character and myself left on the arena.
When the cut-scene ends, I'll have a debuff on me that increases the damage I take from all sources and lowers my movement speed. My attacks will come slower, but mixed in with them is a slash-spell combo that deals immense amounts of damage and will stun on the second hit if both connect. In addition, if the spell from the combo hits, the same debuff from the empowered weapon in the third phase is placed on the character that it hits, preventing them from being healed for ten seconds now, up from its original five seconds. The dying breath of the creature caused pools of liquid to start spawning in random locations about the arena, and if a character stands within one of the pools they'll gain greatly increased health regeneration for the duration of the pool's existence, or until they move out of it. They don't last long while a character is standing in them, and there is a lengthy spawn timer between each one, but they will spawn if another one is still active.
Once I reach 15% health, I drive my sword into the center of the arena that we've been battling on. The remaining ground slowly begins to turn corrupted, dealing increasing damage the longer a character stands inside the corrupted ground. At this point, the final optional quest item comes into play, as it reduces the damage taken from this attack by 75%, because eventually the entire ground will be corrupted. If there are any pieces of wall left from when it crumbled, the corruption will not reach ground behind the walls, however, creating certain 'safe zones' that the player can utilize to reset the amount of damage they take from the corruption, as the pools of healing liquid are still spawning during this phase. The player only has a certain amount of time to finish me off during this final phase, however, because after two or three minutes I pull my sword from the ground again, and the arena surrounding me except for the inner circle I've been standing on completely crumbles away. It is at this point that the player can either move in to the central circle and try finishing me if they've taken this long, now being unable to dodge any of my melee swings and thus being required to kill me before they're killed, or will perish.
Defeating me yields recognition from all significant NPCs within the game, as well as unlocking the 'True' ending of the game, and granting my sword and armor to the player, which carries over into a New Game+. The armor has the highest armor rating in the game, absorbs Shadow and Holy damage and Halves all other Elemental damage, and the sword has a chance-on-swing to send a bolt of corruption through the target, dealing five times normal swing damage every half second for five seconds, as well as having one of the highest base damage outputs of any weapon within the game. It doesn't scale as well with upgrading as some other weapons, however, because of the chance effect.