1. Ranir's death. I understand the humans or western kingdom pulled into the war because of his death. Or they rather begin a vengeance against the undead because of the champion's death? The elven kingdom... why are they pulled into the war because of Ranir's death? Political reasons or something?
A: The elven kingdom is pulled in because it was their camps that were attacked. Ranir just happened to be visiting one of the camps when the attack as part of a political pact (An elven champion was sent to visit the human kingdom as well).
2. The Hero is thrown into the battle and during battle his army is decimated. Sorry if it's just me, but that sentence made me think... when did he acquire an army and how? Why is he a leader of this said army?
A: His army as in the one he was apart of, not necessarily one he is in command of.
3. Martyr is a rather... weird word in my opinion.
A: Martyr was used, to be symbolic of his death on the battlefield making him stronger than he was before. Much like actual martyrs who become a much stronger as a symbol after they die for their cause.
4. His army was being decimated, but he was brought back to the battle with renewed and improved power. In that time I think the army would have been decimated entirely and did he kill off the opposition by himself or did time stop when he was with the cloaked figures.
A: You are correct in that time in the real world stopped during the meeting with the cloaked figures. When he came back he was able to rally the remaining forces, and with his new power and knowledge he was given in parting from the cloaked figures, was able to beat back the enemy.
5. They give him his own army? How many armies do they have? One was just slaughtered(Mostly I would think?) and now there's a new army?
A: Back in medieval times kingdoms would have several armies under the command of the king, or emperor. Each army was led by a general. The confusion I think comes from today's standard is a single army per nation, broken down into smaller components such as companies, platoons and battalions.