When a creature or immortal is able to advance a level this level up button will be highlighted.
You then have to decide if you wish the particular creature to level up or stay where he is at. This Archangel for example starts at level 20. It takes a lot of experience to get to level 21. But even after all this experience do you want to advance a level?
The reason to advance the angel is because it will become more powerful (more health & better stats). However, the mana cost will also increase. (note: mana cost is the amount required to summon the creature rune into play...the archangel costs 690 mana).
So its a balancing act if you think its worth it to advance the creature or not. It depends on your rune army (the other runes you have in your deck).
Unlike creatures, immortals always start at level one. However, they gain a lot more powers/abilities as they advance levels and can become the most powerful characters in the game if they advance enough levels. But to start almost all creatures are more powerful than immortals.
The lower the initiative the sooner the creature plays its turn. Initiative starts at 100 and can go up or down based on speed, size, and certain spell affects and powers.
The Magic Save is very similar to the saving throw used in Dungeons and Dragons. Some spells/powers require that you "save" to avoid certain affects or to take less damage. The higher the better chance to avoid the affect.
You only use the Magic Save VS other creatures/immortals. When you play deity spells there is NO SAVE...as they are a much more powerful type of spell. More on that later though...