Ethereal Swamp
Basic Land - Swamp
{t}: Add {b} to your mana pool.
Whenever you control six or more Ethereal lands, sacrifice all but five of them.
Cycling {2} ({2}, discard this card: Draw a card.)
and
Primeval Swamp
Land - Swamp
Primeval Swamp enters the battlefield tapped.
{t}: Add {b} to your mana pool.
{t}, discard a Swamp: Draw a card.
Sometime I will variants for the other basic lands types as well see how they play out when added liberally to booster draft decks.
Ethereal - Is there much of a point to this drawback? Why "control only five"? Several cycles of lands that tap for one color and have cycling exist, as do the cycling duals of Amonkhet. Although they all enter the battlefield tapped, any deck that wants that amount of cycling can already access it. ETBuntapped cycling lands are possible, but making them A, basic, and B, with a strange, negligible drawback is not the way to go.
Primeval - Do you intend that every color should get one of these? Is there an environment you're creating with custom cards where every color needs to be able to loot? I get that this land can mitigate mana flood, but that's really what the purpose of the mulligan rule is - this is a too slow and conversely too breakable solution to something that can and should be solved other ways.
As a secondary note -- you're referencing the card names as though they're supertypes. You want something like "Basic Ethereal Land - Swamp" for the card to be able to check whether you have six or more Ethereal lands in play.
Basic Land - Swamp
{t}: Add {b} to your mana pool.
Whenever you control six or more Ethereal lands, sacrifice all but five of them.
Cycling {2} ({2}, discard this card: Draw a card.)
and
Primeval Swamp
Land - Swamp
Primeval Swamp enters the battlefield tapped.
{t}: Add {b} to your mana pool.
{t}, discard a Swamp: Draw a card.
Sometime I will variants for the other basic lands types as well see how they play out when added liberally to booster draft decks.
Primeval - Do you intend that every color should get one of these? Is there an environment you're creating with custom cards where every color needs to be able to loot? I get that this land can mitigate mana flood, but that's really what the purpose of the mulligan rule is - this is a too slow and conversely too breakable solution to something that can and should be solved other ways.
I̟̥͍̠ͅn̩͉̣͍̬͚ͅ ̬̬͖t̯̹̞̺͖͓̯̤h̘͍̬e͙̯͈̖̼̮ ̭̬f̺̲̲̪i͙͉̟̩̰r̪̝͚͈̝̥͍̝̲s̼̻͇̘̳͔ͅt̲̺̳̗̜̪̙ ̳̺̥̻͚̗ͅm̜̜̟̰͈͓͎͇o̝̖̮̝͇m̯̻̞̼̫̗͓̤e̩̯̬̮̩n͎̱̪̲̹͖t͇̖s̰̮ͅ,̤̲͙̻̭̻̯̹̰ ̖t̫̙̺̯͖͚̯ͅh͙̯̦̳̗̰̟e͖̪͉̼̯ ̪͕g̞̣͔a̗̦t̬̬͓͙̫̖̭̻e̩̻̯ ̜̖̦̖̤̭͙̬t̞̹̥̪͎͉ͅo͕͚͍͇̲͇͓̺ ̭̬͙͈̣̻t͈͍͙͓̫̖͙̩h̪̬̖̙e̗͈ ̗̬̟̞̺̤͉̯ͅa̦̯͚̙̜̮f͉͙̲̣̞̼t̪̤̞̣͚e̲͉̳̥r͇̪̙͚͓l̥̞̞͎̹̯̹ͅi͓̬f̮̥̬̞͈ͅe͎ ̟̩̤̳̠̯̩̯o̮̘̲p̟͚̣̞͉͓e͍̩̣n͔̼͕͚̜e̬̱d̼̘͎̖̹͍̮̠,͖̺̭̱̮ ̣̲͖̬̪̭̥a̪͚n̟̲̝̤̤̞̗d̘̱̗͇̮͕̳͕͔ ͖̞͉͎t̹̙͎h̰̱͉̗e̪̞̱̝̹̩ͅ ̠̱̩̭̦p̯̙e͓o̳͚̰̯̺̱̰͔̘p̬͎̱̣̼̩͇l̗̟̖͚̠e̱͉͔̱̦̬̟̙ ̖͚̪͔̼̦w̺̖̤̱e͖̗̻̦͓̖̘̜r̭̥e͔̹̫̱͕̦̰͕ ̗͔̠p̠̗͍͍̱̳̠r̰͔͎̰o͉̥͓̰͚̥s̟͚̹̱͔̣t͉̙̳̖͖̪̮r̥̘̥͙̹a͉̟̫̟̳̠̟̭t͈̜̰͈͎e̞̣̭̲̬ ͚̗̯̟͙i͍͖̰̘̦͖͉ṇ̮̻̯̦̲̩͍ ̦̮͚̫̤t͉͖̫͕ͅͅh͙̮̻̘̣̮̼e͕̺ ͙l͕̠͎̰̥i̲͓͉̲g̫̳̟͈͇̖h̠̦̖t͓̯͎̗ ̳̪̘̟̙̩̦o̫̲f̙͔̰̙̠ ̹̪̗͇̯t͖̼̼͉͖̬h̹͇̩e͚̖̺̤͉̹͕̪ ͚͓̭̝̺G͎̗̯̩o̫̯̮̟̮̳̘d̜̲͙̠-̩̳̯̲̗̜P̹̘̥͉̝h͍͈̗̖̝ͅa͍̗̮̼̗r̜̖͇̙̺a̭̺͔̞̳͈o̪̣͓̯̬͙̯̰̗h̖̦͈̥̯͔.͇̣̙̝
Dunes of Zairo
SHANDALAR
Innistrad - The Darkest Night
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A Community Set
Commander: Allies & Adversaries