Ok if I understand how the Archer's abilities work, Pierce when triggered ignores armor (I archer has 100 atk and enemy has 85 armor pierce ignores all armor) the skill Rupture earned with the arrow equipment increases the damage that the archer does when pierce is inflicted. In other words Pierce is then turned into a crit skill.
I can't believe im saying this but the Key to reaching then killing the boss of Cave 20 is mages. With 480 Dark mana used and tons of damage upgrades, I need 20 mages to make the shield spells fast enough to reach the Pit Lord and kill him.
When it comes to the storage on the Spell page, mob drops are used for special spells. These spell have various uses. These special mob drop based spell are not lost when Dark Ritual is used
If you beat the same overseer more then once then each subsequent time gives you a unique worker that boosts wood iron and food by .5% per overseer. This worker persists after Dark Rituals.
Not sure what but oversees will not serve you until you complete a unknown task or reach an unknown point. Iv killed the C13 once didn't get his advantage BUT on the second he was available for use. Same may apply to all Overseers.
When you reach and beat the boss of C15 make sure you beat all the white quests available (green are repeatable). Then time to set your dark mana perks. I recommend you put at least 5 in each of the perks one more in the unit skills and a general skill with the leftovers. Don't put any in mana since that really doesn't advance you as much as the others. Once you've done that click the Dark Ritual button. Note only quest rewards, DM perks, unlocked quests, and global spells (the ones that require mob drops to use).
All right, I will repeat some advise, please up this so people can see. This advise should see you through to Cave 15. Buy 5 Wells as soon as possible, this will give you the mana capacity you will need. Buy 4 Spell factories ASAP this will also be enough to beat C15. Buy Arena when it comes available, subsequently buy all ten Vets by C10 as Cerberus is a fight won by armor. DO NOT FIRE ANY GEM WORKERS. DO NOT BUY MAGES. TRY NOT TO SELL GENERAL WORKERS. Mana regen is not necessary but doesn't hurt.
Quests may not be immediately beatable (fallen priest one is a good example), strengthen you unit(s) and wait till you have battle spells lined up. Some spells are not obtainable till later like Shield Spell (found at C11). This spell when used right can help a lot. DO NOT FRET OVER DARK MANA, this is pointless till you beat the boss of C15.
When it comes to the fallen priests 1 DO NOT ATEMPT WITHOUT SHIELD SPELLS. 2 Have more shields on back production while going through 1x speed (this in general is a great strategy for many situations)
OK so the grind is there and as of Level 15 I have not seen gold or gem idle gain means BUT I did get to keep all artifacts after the Dark Ritual (a huge bonus) and still have access to a green quest (I assume it means repeatable) for food
 "Cave Heroes"){kind=icon}
 "Anti-Idle: The Game"){kind=icon}
 "Cosmic Raiders Alpha"){kind=icon}
 "Realm Grinder"){kind=icon}
 "Idle Raiders"){kind=icon}