Oh hey, its the giant butthole from Stranger Things that they try and close in Season two
It's great if you can snag a good attack trigger along the way, but even if you can't you're still getting a Heat Shimmer effect that leaves behind a mana rock for future turns.
There isn't any good direct comparison to Mangara's abilities, but the closest thing that popped into my head is Runic Armasaur, another creature whose card drawing ability is entirely dependent on your opponents' actions, making the card inconsistent where it sometimes draws you zero cards and sometimes floods you with card draw
The Parts of a Magic Card There are lots of parts of a Magic card that you should familiarize yourself with if you hope to identify cards for pricing
These are substantially better than the original Urza's block ones as they basically cost half to cycle
Second, about the price drops: The value loss is going to be real and big and significant