Summoner War is a smartphone (android & iOS) turn based RPG game. You play for summoner and can summon very much different units (they have 5 different elements!  Game fully F2P, you can get all units for free. (sure there people who spend very big money amount but not much point in it). 
Summoners War: Sky Arena
You can buy energy (i think it most important what you can buy), scrolls to summon hero (3-5 and most time you going get only 3 star hero, also you can get this scrolls in game).  How much crystals you get for free per day? 9*2 from arena (4*2 for 13 level), 7+3 from daily quests that easy to make, 2 from crystal mine and 3 from crystal titan (he cost 300 crystals so you really start to get profit only after 100 days), some times they drop in dungeons and we often have events where you can get them (you can get 170 crystals in events that goes now if you have enough luck) and last place is temple of wishes, you can get there 15-100 crystals if you lucky.
Summoners War: Sky Arena