msem-instigator

When Curious Charter enters, it explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then put the card back or put it into your graveyard.)
As a permanent you control explores a land, you may put it onto the battlefield rather than into your hand.

Reach

Destroy target creature with flying. (Then exile this card. You may cast the creature later from exile.)

Font of Sorrows and other permanents you control entering the battlefield cause their death knell to trigger.
Death knell — Draw a card. (As a creature you control dies, trigger one death knell of that creature or another permanent you control.)

Codex — Majestic Dominion; Pristine Dominion; Verdant Dominion; Stalwart Dominion; Primal Dominion; Rugged Dominion; Into the Woods. (As this enters, conjure its contents into a shuffled face-up additional library.)
Before you explore this library, you may put a card from the codex's library on top of it.
{3}{G}: Target creature you control explores.

When Gemcrusher Turtle enters, choose one —
• Destroy target artifact.
• Gemcrusher Turtle fights up to one target creature.

You gain 3 life. Whenever one or more creatures enter this turn, repeat that process.

Spells and abilities you control that target Inconspicuous Sheep cost {1} less.
As long as Inconspicuous Sheep is enchanted or equipped, they're a 3/3 legendary Sheep Rogue named Agent S.

Creatures you control attacking causes their death knell to trigger.
Death knell — Indomitable Spirit gets +2/+2 and gains trample until end of turn. (As a creature you control dies, trigger one death knell of that creature or another permanent you control.)

Search your library for a Forest card, put it onto the battlefield, then shuffle.

When Little Pig enters, if you control a land named Far-Off Lands, put two +1/+1 counters on it.

Create two tapped colorless land tokens named Far-Off Lands with “{T}: Add one mana of any color.” (Then exile this card. You may cast the creature later from exile.)

Deathtouch, hexproof from non-Adventure
Whenever a creature dealt combat damage by a creature you controlled this turn dies, draw a card.

Sacrifice a creature. Draw cards equal to its power and gain life equal to its toughness. (Then exile this card. You may cast the creature later from exile.)

Hexproof
When Overburdened Trader enters, create three 1/1 colorless Construct artifact creature tokens.

Flash
When Remnant Spotter enters, put target permanent card from your graveyard on top of your library.

Target permanent you control gains hexproof and indestructible until end of turn. You gain 2 life.

Enchant creature or land
Whenever enchanted permanent becomes tapped for the first time each turn, it explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on that creature, then put the card back or put it into your graveyard.)

Choose one —
• Destroy target artifact or enchantment.
• Target creature you control explores twice. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on that creature, then you may put that card in your graveyard.)

You may put cards with Adventures that permanents you control explore into your hand as though they were lands. (You won't get the counter if you do.)
At the beginning of each end step, if you cast an Adventure spell or a permanent you controlled explored this turn, create a 1/1 green Scout creature token.

When Something in the Woods enters or at the beginning of your upkeep, Something in the Woods explores. If it explores a creature card, you may have it become a copy of that card. (It loses this ability. To explore, reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on this, then you may put that card into your graveyard.)

When Sorrowslayer enters, put a +1/+1 counter on target creature.
Remove a +1/+1 counter from Sorrowslayer: Trigger a death knell of a permanent you control. Activate only once each turn and only as a sorcery.

Trample
Death knell — Put a +1/+1 counter on Tempest Dodger. (As a creature you control dies, trigger one death knell of that creature or another permanent you control.)

Vigilance
When Tortle Guard enters, it explores. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then you may put that card in your graveyard.)

{G}, {T}: Add one mana of any color. When a creature enters this turn, if that mana was spent to cast it, it explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then put the card back or put it into your graveyard.)