They provide 20k permanent bonus to each stat. If you collect ten of all four types of either the silver set or blue set (800k to each stat, total; 40 total idols), then they combine into a single new idol worth 1m in each stat. They drop randomly from event chests, this is I believe the third consecutive event they've been dropping.
Do they increase again after the first level if you collect another 40 or do the nxt 40 just turn in to another separate entity?!
The 40 combine for 1 beasts of the hoarfrosts, or 1 riders of the lowlands depending on the set. Then the next 40 will make another one and so forth. But there is only 1 level per combo. They are worth 4m total stat boost each.