My idea for a legendary secret

You discover the effect at the start of the game, but still have to draw and play the secret...

They are not gonna get us.

Pendulum Blade fits in nicely with the Dungeon Run traps theme which Pressure Plate started. AOE is one of Hunter's weaknesses, so the effect is well worthy of that legendary gem but still conditional enough not to be completely broken. Flavor-wise, the Pendulum Blade passes over the heads of smaller minions, so only the big ones get killed. I hope you like it! 

Disruption or combo trickery? Why not both.

Reckoning of Drunhei provides Hunters a form of protection similar to Ice Block, stopping any oncoming attacks and protect your hero at low Health, summoning a huge 10/10 Taunt minion with a lot of protection that can become a bigger problem if not dealt with. It however requires you to drop to a low life total to summon it and will not prevent the opponent from killing you through other means. As the Secret will stay on your hero until you take fatal damage, it can also be a long-term way to activate "if you control a Secret" effects.

The name of Drunhei is a reference to Heidrun, a goat in Norse mythology.

"Boss, I've got it right here - our opponent is trying to 'guard Ian Animals' - whoever that is!"

The Top Secret! for the top secretive class of the game.

Insane potential, but also possible for the opponent to play around.
Not too op for its cost, thanks to the randomness, but still confusing your opponent and giving you at least 3 mana worth of value.

"An Eye for an Eye makes the whole world blind."

A super Mirror Image. Allow the class to have some defensive measure against board flood decks.

Because if I'm not having fun, neither are they.

The name comes from what Vision says in the movie Captain America: Civil War: "Our very strength invites challenge. Challenge incites conflict. And conflict... breeds catastrophe."

The idea being that the strength of the opponent encourages your minions to rise up and Avenge the damage inflicted upon you. Might be a bit on the slow side, but with the right deck (perhaps one heavy with Rush and/or Taunt minions), you can retake the board and really start to benefit from your buff.

The main idea about Cauterize is, like the original version of wow's mage spell, to gain combo potential at the cost of surviving.

The wow ability makes you inmune to snares/slows, gives you a movent boost and avoids you taking lethal damage at the cost of receiving damage overtime which will in all reality, eventually kill you.

Here, the idea is, if your opponent is going to push damage your way, and isn't able to kill you, you can retaliate hard! 

Great if you draw a lot, but only if the game lasts long enough, or you play Myra's Unstable Element. Also, you have to have much more draw than your opponent, or you won't get many cards

Fantastic treasures - Tolin's Goblet, Golden Kobold, Wondrous Wand, Zarog's Crown

A rogue's path to riches is getting someone else to do all the work and taking the treasure for yourself.

'What kind of a rube do you take me for?'

I know, this is pretty much Ice Block but instead of becoming immune, it hits the enemy hero for a possible counter lethal.

Just to make clear, your health gets set to and is capped at 1

Like Explosive Runes, any battlecry effects will still go off. A niche effect that might not always come into play, but damn would it be nice against Pen Flinger.

So if you’re at 5 Health and your opponent deals 5 to face, this triggers and you’re still at 5 Health, but now if your opponent can’t deal 5 damage again they die. This forces your opponent to think twice before throwing everything they have into lethal. It can buy you some survivability and requires no setup, but since it can be played around fairly simply I think it’s balanced.  

So if your hero took, say, 4 damage on the opponent's turn, you'll have 4 extra Mana Crystals on the next turn. I added "can exceed 10" to clarify that you're not bound to the 10-Mana limit, like The Coin, Innervate and Lightning Bloom are.

"Ever since the Horde moved into the ruins of Azjol-Nerub, the number of legs has been decreasing by a factor of eight."

A powerful Secret that brings back a favourite design of mine: Spider/Nerubian Rogue. Flavour-wise, the card is meant to evoke the Nerubians rising up to reclaim their long-lost empire.

Note: This would work like Plagiarize, and would not trigger if your opponent didn't play any cards in a turn.

A secret for control Rogue decks that want some protection against token decks amd more aggressive strategies in general.
Can be a bit confusing for the opponent as it uses a pretty uncommon condition for the trigger.

Having the trigger on your opponent's 5th minion on the board may seem like much but having it any lower would make for a pretty toxic design (as it would board-lock some decks pretty hard). Opponents can also, sometimes, play around this by keeping 4 minions on the board thus keeping the Rogue from developing a board as [Hearthstone Card (Vanish/card] is a symmetrical effect.