1 Talisman. Wherever you get it (if you don't get any on your current battle, it carry overs). You can use it multiple times to have more uses stacked (it is not unusual to get 3 talismans on the later levels).
It is very worth after you are getting over 500 cores per level. Before that it is kinda nice, but not really needed.
Your initial mana can be raised by some things, talisman fragments for expamle, also unused skillpoints give you some initial mana, so dont spent all of your skillppints in skills... lvl up to get mote skillpoints (replay levels, if possible on higher difficulty) and try to get achivements.
