When Third Legion Rider enters, if you control an embattled creature, tap target creature an opponent controls. It doesn't untap during its controller's next untap step. (A creature is embattled if it has a counter on it or has a permanent you control attached to it.)
When Thoughtgrinder enters, each player discards a card. Unearth {1}{U}{B}({1}{U}{B}: Return this card from your graveyard to the battlefield. It gains haste. Exile it at the beginning of the end step or if it would leave the battlefield. Unearth only as a sorcery.)
Target creature gets +2/+0 and gains haste until end of turn. Fabled {2}{R}(Then if you cast this for its fabled cost, put it onto the battlefield transformed attached to that creature.)
Target creature gets +1/+1 until end of turn for each creature you control. Inscribe {3}{R}({3}{R}: Exile this card from your hand inscribed on a creature you control. Whenever that creature attacks, its controller may cast a copy of the inscribed card without paying its mana cost. Inscribe only as a sorcery.)