Explanations

IS TO BE CAST...= Must be thrown...
The form TO BE + Infinitive may be used to express strong obligation:
- You are to be here by 10 = You definitely must be here at 10 or before
But this form is often used in newspaper headlines to simply express a fixed plan in the future (often omitting the verb to be):
- The Queen (is) to inaugurate the new bridge next Monday.

BACK= This particle expresses that the action of the verb goes back to the origin. The sentence "the ring's to be cast back into the fires of Mount Doom" means that the ring came from that mountain and now it must go back there. "The fires of Mount Doom" means that that mountain is a volcano. The word DOOM means "fate, destiny", but "The Doom's Day" is what the Bible also calls the Day of Judgement, the end of the world, and though the Bible doesn't imply that on that day the world will be destroyed forever (actually, the old world/civilization will be destroyed to give rise to a new one), nevertheless, people usually identify the Doom's Day with total destruction, and so is the idea we get when we hear the "Mount of Doom".

CANNOT= Notice that the negative of CAN is CAN'T (informal English) or CANNOT (formal English), but spelled in just one word and pronounced /kænət/.

FELLOWSHIP= A fellowship is a group of people who share the same interests and objectives and usually work together for that goal.

SWORD= /sɔ:*d/ (see picture)

BOW= /bəʊ/ (see picture)

AXE= /æks/ (see picture)

VUVUCELA= A horrible sounding horn that was popular in South Africa and got worldwide famous due to South Africa's Football World Cup celebration in 2010. The sound is so disturbing and distracting that the FIFA decided to forbid its use in football games throughout Europe. You can see vuvucelas used on the video.

WITHIN= inside.

MIDDLE EARTH= It's one of the kingdoms in the imaginary world of "The Lord of The Rings", the movie this video is based on.

SHALL= An emphatic form of the auxiliary verb WILL, often used to state a promise.

PREVAIL= (formal) If you prevail in a conflict or a fight, you win.

MAN= (informal, esp. AmE) familiar form of address for a man:
- Hey man, how you doin'?
- I'm fine, man, how 'bout ya?

RUINED= Spoiled, broke.