Eternia Crystal

This is how you start the Old One's Army. Put down an Eternia Crystal Stand, hit it while carrying an Eternia Crystal, and the crystal drops into the stand. It refuses if the ground is not right: you need a long flat run to either side of the column you clicked, roughly sixty tiles each way, and if you are short it pops straight back out with a message rather than failing quietly. Level the ground first.

Once it takes, two portals open a good distance out on both sides, there is a five second countdown, and then everything that comes through heads for the crystal rather than for you. Lose the crystal and the event ends in failure on the spot. Keep it standing through the last wave and you win.

Placing it inflicts Creative Shock on everyone in the area for the whole event, which means no placing and no mining until it is over. The arena you meant to build has to exist beforehand. Walls, platforms, campfires, the lot. The Ice Rod is the one exception that still functions, but ice will not fence anything out, because the attackers pass through terrain anyway.

Lightning is the one that catches people out. Run the event during a thunderstorm and a handful of strikes will finish the crystal at the lowest tier, on every difficulty, and you lose to the weather rather than to the invasion. Roof it with solid blocks, drop temporary ice over it while a strike builds, or just hold the whole event underground where the sky cannot reach it.

Explosives are the other trap. The crystal itself shrugs them off, but the stand and the ground under it do not, so a stray blast leaves it falling until it lands on something solid, usually somewhere much less convenient. Rotten Eggs damage it directly, which is worth remembering if you were planning to use them as a weapon.

Its health and defense are tied to progression: one value before you have killed a mechanical boss, another after, another after Golem. None of that makes the later tiers gentler, because everything attacking it scales at the same time, arrives in bigger numbers and hits harder. Do not read the tougher crystal as a cushion.

The crystal can be moved with hoiks after placement, and since you cannot touch a block for the duration, that is the only way to reposition anything mid event. It opens defensive layouts a fixed crystal cannot use.