As with most early beta releases, Dota Underlords is proving to be a bit of a balancing challenge for Valve. The latest exploit discovered by players relates to Arc Warden's cloning ability, provided the clones can be fed with mana.
Valve are still scrambling to balance the numerous exploits players are digging up with each patch and update Dota Underlords receives. The game is admittedly still very early into its beta phase and what can be called the current meta is in a state of flux.
The culprit currently responsible for ruining the game's internal harmony is an Arc Warden / Crystal Maiden combo.
The developers were probably aware of a potential exploit here and have intentionally designed Arc Warden's skill so that it doesn't trigger an infinite clone-fest by making copies of the hero and whatever item he is carrying less efficient in gaining mana, but players have found a way around that.
The Crystal Maiden hero passively helps all heroes charge mana faster, and coupled with any number of items which affect mana regeneration and cooldowns, Arc Warden hordes have become a common sight.
To make matters even worse, items such as Blink Dagger help the Primordial Shaman type Arc Warden get his clones out of trouble long enough to create another clone. When combined with Alliance bonus items that have a chance of turning heroes attacking Shamans into a chicken, Arc Wardens become a lot more effective when a player has a great number of them. That's without even bringing up their ability to potentially disarm opponents if another Primordial hero is on the field.
Oh, almost forgot, every clone surviving at the end of a round also deals round damage, which can lead to some pretty quick knock-out games.
Long story short - Arc Warden is OP and Valve are aware of this.
A fix for the current state of affairs is likely to arrive before the usual weekly update, but there is still no word on when exactly.
Some players are also pointing fingers in the direction of Crystal Maiden, rather than Arc Warden as the target for the inevitable nerf, as her ability helps a lot of glass cannons get their spells out faster.
While this may be true, it leaves players with more than enough avenues to keep Arc Warden spamming clones, such as the aforementioned Blink Dagger, Brooch of the Aggressor, Arcane Boots or Refresher Orb - all of which can help his mana situation.
Currently, the best way to deal with this seems to be relying on a strong Assassin line and hoping that it will take at least one component of this infernal combo out of the round, or finding ways to silence the offending Arc Warden before getting snowballed.
Regular offerings to the fickle deities of RNG might also do the trick, as long as you make sure the pentagrams aren't drawn with a permanent marker.