Merlin

Sir Leon gets the last line of the show.

It is revealed that Merlin is immortal.

Goof: Arthur isn't wearing a cloak at any point during the episode. However, when Merlin places him on the boat he has his Camelot's cloak on.

Goof: When Merlin and Arthur get off the dragon, they are close to the lake shore and there's a boat behind them. However, the distance to the shore changes between takes and the boat appears and disappears.

Sir Gwaine is killed by Morgana in this episode becoming the 3rd named Knight of the Round Table to die after Sir Lancelot (who sacrificed himself for Arthur in episode 4x02 The Darkest Hour Part 2) and Sir Elyan (who fought to the death against a enchanted sword to rescue Gwen in 5x06 The Dark Tower).
Mordred, Morgana and Arthur also die in this episode.

Arthur is the third dear person to Merlin that he has to send adrift on a boat into a lake. He gave burial to Freya in this fashion in 2x09 The Lady of The Lake and to Lancelot (Lancelot's shadow) in 4x09 Lancelot Du Lac. Freya and Lancelot (and Sir Elyan in 5x06 The Dark Tower) were set on fire unlike Arthur, who is profe to rise again.

Just like it happened when he had to part from Freya, Merlin hesitates before enchanting the spell that sets Arthur's boat into motion.

Since Arthur is struck by a blade forged in a dragon's breath it would take a power as ancient as the dragons themselves to revert the fatal consequences. Only the Sidhe possess such magic and the source of their power resides in an ancient isle in the midst of the Lake of Avalon.

Merlin's line 'You know I was betrayed, the girl Eira can not be trusted' is clearly some additional voice recording as it doesn't match Merlin's facial movements or overall sound of the scene.

Goof: Percival has no marks whatsoever on his skin after having been bound with ropes and having freed himself by snapping them torn.
Somehow he recovers his armbands later on in the episode.

This episode is 5 minutes longer than the usual length of the show as was episode 2x13 The Last Dragonlord.

Merlin addresses the Great Dragon as Kilgharrah for the second time after episode 4x04 Aithusa since Balinor revealed the dragon's name in episode 2x13 The Last Dragonlord.

This episode is the sixth time in which Arthur is referred to as The Once and Future King (by the Great Dragon). The Great Dragon did so in 1x01 The Dragon's Call, The Fisher King in 3x08 The Eye Of The Phoenix, Merlin in 4x11 The Hunter's Heart, Osgar in 5x05 The Disir and Finna in 5x10 The Kindness Of Strangers.

The Great Dragon tells Merlin that 'No man, no matter how great, can know his destiny', echoing the opening line of the show ('No young man, no matter how great, can know his destiny') as heard in episode 1x01 The Dragon's Call.

The Nathair serpent that Morgana uses on Gwaine was previously featured in episode 4x12 The Sword in the Stone Part 1 (where it was used on Elyan) and in episode 5x10 The Kindness Of Strangers (where it was used on Alator of the Catha).

This episode marks the 6th appearance of Excalibur. It was first seen in episode 1x09 Excalibur, in episode 3x13 The Coming Of Arthur, Part 2, in episode 4x13 The Sword In The Stone, Part 2, 5x09 With All My Heart and 5x12 The Diamond Of The Day Part 1.
The sword has not been referred to as 'Excalibur' in either. The sword is returned to the Lady of the Lake.

The intro for this episode consists of previously seen scenes from 5x12 The Diamond Of The Day Part 1.

This episode marks the third time in which the Dochraid can be seen after 4x09 Lancelot Du Lac and 5x09 With All My Heart.
It also marks the return of Balinor who was first seen in 2x13 The Last Dragonlord.

Balinor refers to Merlin as a 'son of the earth, the sea, the sky'. The Dochraid defined herself as 'a creature of the earth' in episode 5x09 With All My Heart.

This episode marks the second time in which the Crystal Cave is visited after 3x05 The Crystal Cave.

Goof: Gaius wears glasses, even though they weren't commonly used until the 13th century.