Chittering Swarmling

Echo {b} (At the beginning of your upkeep, if this
came under your control since the beginning of your
last upkeep, sacrifice it unless you pay its echo cost.)
When Chittering Swarmling enters the
battlefield, target player mills a card for each Rat
you control.
When you pay Chittering Swarmling's echo cost,
create a token copy of Chittering Swarmling.