Whirler Virtuoso

When Whirler Virtuoso enters the battlefield, you get {E}{E}{E} (three energy counters).
Pay {E}{E}{E}: Create a 1/1 colorless Thopter artifact creature token with flying.

Solar Transformer

Solar Transformer enters the battlefield tapped.
When Solar Transformer enters the battlefield, you get {E}{E}{E} (three energy counters).
{T}: Add {C}.
{T}, Pay {E}: Add one mana of any color.

Guide of Souls

Whenever another creature enters the battlefield under your control, you gain 1 life and get {E} (an energy counter).
Whenever you attack, you may pay {E}{E}{E}. When you do, put two +1/+1 counters and a flying counter on target attacking creature. It becomes an Angel in addition to its other types.

Satya, Aetherflux Genius

Menace, haste
Whenever Satya, Aetherflux Genius attacks, create a tapped and attacking token that's a copy of up to one other target nontoken creature you control. You get {E}{E} (two energy counters). At the beginning of the next end step, sacrifice that token unless you pay an amount of {E} equal to its mana value.

T-45 Power Armor

When T-45 Power Armor enters the battlefield, you get {E}{E} (two energy counters).
Equipped creature gets +3/+3 and doesn't untap during its controller's untap step.
At the beginning of your upkeep, you may pay {E}. If you do, untap equipped creature, then put your choice of a menace, trample, or lifelink counter on it.
Equip {3}

Roil Cartographer

Landfall — Whenever a land enters the battlefield under your control, you get {E} (an energy counter).
{T}, Pay six {E}: Draw three cards.

Glimmer of Genius

Scry 2, then draw two cards. You get {E}{E} (two energy counters).

Rex, Cyber-Hound

Whenever Rex, Cyber-Hound deals combat damage to a player, they mill two cards and you get {E}{E} (two energy counters).
Pay {E}{E}: Choose target creature card in a graveyard. Exile it with a brain counter on it. Activate only as a sorcery.
Rex has all activated abilities of all cards in exile with brain counters on them.

Gonti's Aether Heart

Whenever Gonti's Aether Heart or another artifact enters the battlefield under your control, you get {E}{E} (two energy counters).
Pay eight {E}, Exile Gonti's Aether Heart: Take an extra turn after this one.

Localized Destruction

You get {E} (an energy counter), then you may pay one or more {E}. If you do, each creature you control with power equal to the amount of {E} paid this way gains indestructible until end of turn.
Destroy all creatures.

Aether Hub

When Aether Hub enters the battlefield, you get {E} (an energy counter).
{T}: Add {C}.
{T}, Pay {E}: Add one mana of any color.

Cyclops Superconductor

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
When Cyclops Superconductor enters the battlefield, you get {E}{E}{E} (three energy counters).
When Cyclops Superconductor dies, you may pay {E}{E}{E}. When you do, Cyclops Superconductor deals damage equal to its power to any target.

Confiscation Coup

Choose target artifact or creature. You get {E}{E}{E}{E} (four energy counters), then you may pay an amount of {E} equal to that permanent's mana value. If you do, gain control of it.

Consulate Surveillance

When Consulate Surveillance enters the battlefield, you get {E}{E}{E}{E} (four energy counters).
Pay {E}{E}: Prevent all damage that would be dealt to you this turn by a source of your choice.

Galvanic Discharge

Choose target creature or planeswalker. You get {E}{E}{E} (three energy counters), then you may pay any amount of {E}. Galvanic Discharge deals that much damage to that permanent.

Inventor's Axe

Flash
When Inventor's Axe enters the battlefield, you get {E}{E} (two energy counters).
When Inventor's Axe enters the battlefield, attach it to target creature you control.
Equipped creature gets +2/+0.
Equip—Pay {E}{E}.

Decoction Module

Whenever a creature enters the battlefield under your control, you get {E} (an energy counter).
{4}, {T}: Return target creature you control to its owner's hand.

Vault 112: Sadistic Simulation

(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.)
I, II — Tap up to one target creature and put a stun counter on it. You get {E}{E} (two energy counters).
III — Pay any amount of {E}. If you paid one or more {E} this way, shuffle your library, then exile that many cards from the top. You may play one of those cards without paying its mana cost.

Aether Theorist

When Aether Theorist enters the battlefield, you get {E}{E}{E} (three energy counters).
{T}, Pay {E}: Scry 1. (Look at the top card of your library. You may put that card on the bottom of your library.)

Fabrication Module

Whenever you get one or more {E} (energy counters), put a +1/+1 counter on target creature you control.
{4}, {T}: You get {E}.

The Motherlode, Excavator

When The Motherlode, Excavator enters the battlefield, choose target opponent. You get an amount of {E} (energy counters) equal to the number of nonbasic lands that player controls.
Whenever The Motherlode attacks, you may pay {E}{E}{E}{E}. When you do, destroy target nonbasic land defending player controls, and creatures that player controls without flying can't block this turn.

Wrath of the Skies

You get X {E} (energy counters), then you may pay any amount of {E}. Destroy each artifact, creature, and enchantment with mana value less than or equal to the amount of {E} paid this way.

Behemoth of Vault 0

Trample
When Behemoth of Vault 0 enters the battlefield, you get {E}{E}{E}{E} (four energy counters).
When Behemoth of Vault 0 dies, you may pay an amount of {E} equal to target nonland permanent's mana value. When you do, destroy that permanent.

Plasma Caster

Equipped creature gets +1/+1.
Whenever equipped creature attacks, you get {E}{E} (two energy counters).
Pay {E}{E}: Choose target creature that's blocking equipped creature. Flip a coin. If you win the flip, exile the chosen creature. Otherwise, Plasma Caster deals 1 damage to it.
Equip {2}

Razorfield Ripper

Whenever Razorfield Ripper or equipped creature attacks, you get {E} (an energy counter), then it gets +X/+X until end of turn, where X is the amount of {E} you have.
Reconfigure—Pay {2} or {E}{E}{E}. (Pay {2} or {E}{E}{E}: Attach to target creature you control; or unattach from a creature. Reconfigure only as a sorcery. While attached, this isn't a creature.)

Aether Chaser

First strike
When Aether Chaser enters the battlefield, you get {E}{E} (two energy counters).
Whenever Aether Chaser attacks, you may pay {E}{E}. If you do, create a 1/1 colorless Servo artifact creature token.

Dynavolt Tower

Whenever you cast an instant or sorcery spell, you get {E}{E} (two energy counters).
{T}, Pay {E}{E}{E}{E}{E}: Dynavolt Tower deals 3 damage to any target.

Spontaneous Artist

When Spontaneous Artist enters the battlefield, you get {E} (an energy counter).
Pay {E}: Target creature gains haste until end of turn.

HELIOS One

{T}: Add {C}.
{1}, {T}: You get {E} (an energy counter).
{3}, {T}, Pay X {E}, Sacrifice HELIOS One: Destroy target nonland permanent with mana value X. Activate only as a sorcery.

Electrosiphon

Counter target spell. You get an amount of {E} (energy counters) equal to its mana value.

Lightning Runner

Double strike, haste
Whenever Lightning Runner attacks, you get {E}{E} (two energy counters), then you may pay eight {E}. If you pay, untap all creatures you control, and after this phase, there is an additional combat phase.

Emissary of Soulfire

When Emissary of Soulfire enters the battlefield, you get {E}{E}{E} (three energy counters).
Pay {E}{E}: Put an exalted counter on target creature you control. Activate only as a sorcery. (Whenever a creature you control attacks alone, it gets +1/+1 until end for each instance of exalted among permanents you control.)

Izzet Generatorium

If you would get one or more {E} (energy counters), you get that many plus one {E} instead.
{T}: Draw a card. Activate only if you've paid or lost four or more {E} this turn.

Aether Inspector

Vigilance
When Aether Inspector enters the battlefield, you get {E}{E} (two energy counters).
Whenever Aether Inspector attacks, you may pay {E}{E}. If you do, create a 1/1 colorless Servo artifact creature token.

Aetherworks Marvel

Whenever a permanent you control is put into a graveyard, you get {E} (an energy counter).
{T}, Pay six {E}: Look at the top six cards of your library. You may cast a spell from among them without paying its mana cost. Put the rest on the bottom of your library in a random order.

Consulate Turret

{T}: You get {E} (an energy counter).
{T}, Pay {E}{E}{E}: Consulate Turret deals 2 damage to target player or planeswalker.

Janjeet Sentry

When Janjeet Sentry enters the battlefield, you get {E}{E} (two energy counters).
{T}, Pay {E}{E}: You may tap or untap target artifact or creature.

Robobrain War Mind

Robobrain War Mind's power is equal to the number of cards in your hand.
When Robobrain War Mind enters the battlefield, you get an amount of {E} (energy counters) equal to the number of artifact creatures you control.
Whenever Robobrain War Mind attacks, you may pay {E}{E}{E}. If you do, draw a card.

Sphinx of the Revelation

Flying, lifelink
Whenever you gain life, you get that many {E} (energy counters).
{W}{U}{U}, {T}, Pay X {E}: Draw X cards.

Phyrexian Ironworks

Whenever you attack, you get {E} (an energy counter).
{T}, Pay {E}{E}{E}: Create a 3/3 colorless Phyrexian Golem artifact creature token. Activate only as a sorcery.

Scurry of Gremlins

When Scurry of Gremlins enters the battlefield, create two 1/1 red Gremlin creature tokens. Then you get an amount of {E} (energy counters) equal to the number of creatures you control.
Pay {E}{E}{E}{E}: Creatures you control get +1/+0 and gain haste until end of turn.

Stone Idol Generator

Whenever a creature you control attacks, you get {E} (an energy counter).
{T}, Pay six {E}: Create a 6/12 colorless Construct artifact creature token with trample. Activate only as a sorcery.

Conversion Apparatus

{T}: Add {C}.
{3}, {T}: You get {E}{E}{E} (three energy counters).
{T}, Pay {E}{E}{E}: Add three mana in any combination of colors.

Brotherhood Scribe

Metalcraft — {T}: You get {E} (an energy counter). Activate only if you control three or more artifacts.
Whenever you get one or more {E} during your turn, creatures you control get +1/+1 until end of turn.

Aethertorch Renegade

When Aethertorch Renegade enters the battlefield, you get {E}{E}{E}{E} (four energy counters).
{T}, Pay {E}{E}: Aethertorch Renegade deals 1 damage to target creature.
{T}, Pay eight {E}: Aethertorch Renegade deals 6 damage to target player or planeswalker.