Onyx Sentinel is the very first card that I get from my daily quest, and to be honest as someone who is just started to play Splinterlands, a card with no attack and low speed is really a downer to get, but since I started to play properly and learn the ropes my rating started to increase and with the increase of rating come the rule/condition during the games.
I will try and create a different post explaining the different types of rules/condition.
Going back, I just started to see the potential of this card, as I encountered the condition "Reversed Speed" as I am using Brighton Bloom, it gives my monsters the "Flying" ability which increase the card's chances to evade attacks from cards without the "Flying" Ability. From my research Evasion is affected by the speed of each card and with the "Reversed Speed" condition my Onyx Sentinel this game was able to tank the damage the entire game, because of its slow speed its evasion rate in "Reversed Speed" is really good and if you add the "Flying" ability too it the Onyx Sentinel is just monstrous.