Until the end of this turn, all DARK Spellcaster-Type monsters with 2000 or more ATK you currently control cannot be destroyed by battle, their effects are negated, also they are unaffected by your opponent's card effects.
Unaffected by Monster Cards / Unaffected by Spell Cards / Unaffected by Trap Cards / Negates the effects of your Effect Monsters
Cannot be destroyed by battle
To protect important monsters (like Dark Cavalry), I still prefer Forbidden Lance because of its speed and versatility. Not seldom, 800 ATK make the difference between you losing your monster or your opponent losing his whereas Black Illusion would just protect your monster one round but not destroy the enemy monster with more ATK.
It´s sad to hear nowadays that trap cards are way too slow for competitive play. Barely none of the top decks at TCG in recent years had even one of them. Only way to change players´ minds was to start making traps you can activate from your hand.
Let´s try to enjoy these cards while we still can! Even though it´s a trap card, it makes DM immune to other cards effects. Give it a chance at least.