When it ETbs, you choose a creature type and then it arrives with your number of those you control in counters and that kindred type is increased in size by the counters
Thief of Sanity
The reward is flexible, giving you treasures that can either be used to keep up the tempo by providing you with more mana for spells or saved to provide solid flying bodies, with haste, later
Two mana: deal 1 damage to a player and draw a card is one of the best activated abilities of all time, given how cheap it is
The Core backside of Matzalantli, the Great Door seems more reliably powerful than the Gaea's Cradle backside that is Itlimoc, Cradle of the Sun unless you are in elfball, but its harder to flip and the front side doesnt do quite enough
Sadly, this effect only triggers once per turn, but that is easily compensated for by cracking lands on each players turn