Sludge Monster - ***** counter not effective after Sludge Monster dies
Bug: When my Sludge Monster is in play, ***** counters on opponent's creatures turns them into 2/2 without abilities (text on card is greyed out). When opponent kills my Sludge Monster, ***** counters are still attached to opponent's creatures, but they have no effect (creature reverts to original stats and text is no longer greyed out).

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Rezzahan#77802 commented
That's because the ***** counter has no effect of its own. It is just a marker, like most other counters. What removes the abilities and makes the creature a 2/2 is the static ability of the Sludge Monster itself. Hence why the creature reverts to normal as soon as there is no Sludge Monster on the battlefield. And goes back to being a 2/2 with no abilities when another Sludge Monster enters the battelfield.