I've seen this clip (and thought it was pretty good) ... but haven't listened to the entire album. Not sure if it's a fair portrayal of Charlemagne or if it's anti-Christian.
I was skeptical too, but, after listening to about half of the more symphonic first album, I can say it is definitely not anti-Christian
From Act I: King of the Franks:
Charlemagne:
Father, does my brother Carloman wait for me?
And my mother Bertha who, with good will and diplomacy
Did all in her power to keep the peace
How was Aquitaine?
The first uprising of my reign
I ran those usurping traitors into the sea.
Did I please Our Lord Jesus when I united ourpeoples in His Faith?
The Saxons ever a plague on our lands, the Lombards that vile race
They would have desecrated his Church and all that we hold dear
But I led them to redemption in Christ the Saviour.
Oh Father, this oath I swear that I shall fight to keep the Lord
In all of Francia and beyond
To heathen fields and Saxon hordes
Who would tread upon a sacred place of ours
Shall learn to fear the name of Charlemagne
One that will be without peer even long after I have gone.
Seems like a pretty pious handling of the source material. And, if I recall, Wiki says Lee was an "Anglo-Catholic", so he wasn't an enemy of the Faith, as far as I can tell.
Fun fact from the comments of your video (source unknown):
"Christopher Lee is a direct descendent of Charlemagne, the first Holy Roman Emperor. Lee's family bears 80-90% of his coat of arms, and his series of concept records are partially meant to clear Charlemagne's name, who is generally referred to as the brutal conquerer of the Saxons in Northwest Germany"