Deathless Ancient — ruling 1

Deathless Ancient's last ability can be activated only while it's in your graveyard.

Deathless Ancient — ruling 2

Once you announce that you're activating Deathless Ancient's last ability, no player may take other actions until the ability's been paid for. Notably, players can't try to stop the ability by tapping or removing Vampires you control.

Deathless Ancient — ruling 3

To activate the last ability, you may tap any untapped Vampires you control, including ones you haven't controlled continuously since the beginning of your most recent turn. (Note that tapping the creature doesn't use {T} [the tap symbol].)

Deathless Ancient : règles, fonctionnement et interactions (MTG)

Deathless Ancient is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying Tap three untapped Vampires you control: Return this card from your graveyard to your hand.

How it works : Deathless Ancient

Original English text — Wizards of the Coast / Scryfall

#1September 29, 2017· Wizards of the Coast
Deathless Ancient's last ability can be activated only while it's in your graveyard.
No other card is named in this ruling.
#2September 29, 2017· Wizards of the Coast
Once you announce that you're activating Deathless Ancient's last ability, no player may take other actions until the ability's been paid for. Notably, players can't try to stop the ability by tapping or removing Vampires you control.
No other card is named in this ruling.
#3September 29, 2017· Wizards of the Coast
To activate the last ability, you may tap any untapped Vampires you control, including ones you haven't controlled continuously since the beginning of your most recent turn. (Note that tapping the creature doesn't use [the tap symbol].)
No other card is named in this ruling.