Besides creatures that make more creatures, ideal reanimation targets are creatures that have a big impact the moment they hit the battlefield, so that even an opponent holding Swords to Plowshares with available will have a hard time recovering from your unfairness
Illusionary Mask disguises our creatures (within their mana value) and, while we have to flip them up when they attack or do anything else meaningful, its still less information for our opponents to go off of
In a different color combination that would not have been much of an issue as control decks would gladly use a card like this to buy themselves time, and perhaps would do so now that colored mana is plentiful, but the beatdown decks that were in these colors were understandably reluctant to force themselves to count to twenty-five
This is where mana ramp comes in
Shadowheart, Dark Juusticiar Shadowheart, Dark Justiciar gets everything that the Half-Elf Cleric is all about in terms of flavor, from the Black alignment to the shoutout to Shar in the lore text