Skeletal Swarming is a rare Enchantment card from Magic Online Promos (PRM), collector number 92818.
Each Skeleton you control has trample, attacks each combat if able, and gets +X/+0, where X is the number of other Skeletons you control. At the beginning of your end step, create a tapped 1/1 black Skeleton creature token. If a creature died this turn, create two of those tokens instead.