During Recruitment events, you are able to use soul stones (which come in large and small forms) to make various monster units into usable hero units. Large soulstones (LSS) are often awarded as prizes for events, and also typically come as a bonus with boxes during the recruitment event. Small soulstones (SSS) can be found in chests during training. Both types can be traded, but be aware that within trades it doesn't tell you which type is being traded, so make sure you know which is which (LSS are lighter in color)!
Each recruitment attempts cost 10K luna.
How many SS it takes to recruit a monster differs depending on 1) the monster's rarity (higher rarity = more difficult), 2) the monster's level (higher level gives a slight discount to number of SS used), 3) type of SS used (LSS are worth about ~40 SSS, so it takes less of those to recruit). When a monster is recruited, it turns into a level 1 unit, regardless of what level the monster was before.
When recruiting, Mina will tell you what she thinks the success rate will be. There are different phrases she will use to indicate success chance. Be aware that even the highest possible success rate is not 100%. You can still fail even if Mina is telling you that recruitment is very likely to succeed!
In the event that you fail, you will lose the luna and SS used, but keep the monster card.Mina's Messages (Percentages are guesswork.)
I have little faith your offering will be successful. — 0 to 9%
Hmm, improbable, but not impossible. — 10% to 24%
Well, we can always hope for the best. — 25% to 39%
Who knows? Your guess is as good as mine! — 40% to 59%
I have a good feeling about this. — 60% to 79%
I think our odds for success are extremely high. — 80% to 100%
5* Success Rate (Guesswork formula, by gtmeteor)
=round(( 4*SSL + 0.1*SSS) * (1 + LVL/300))
6* Success Rate
4* Recruitment Chances (Credits to themadmanazn)
0-11 : I have little faith that your offer will be successful.
12-29 : Hmm, improbable, but not impossible.
30-47 : Well, we can always hope for the best.
48-71 : Who knows? Your guess is as good as mine!
72-95 : I have a good feeling about this.
96+ : I think our odds for success are extremely high. NOTE: You cannot reach a 100% in recruiting! You might still fail even though you have more than 90% chance.