Sinn Fein's spectacular success in the Dail election is the stuff of DUP nightmares.
Arlene Foster's party knows all too well from 2017 how an election can transform a party's fortunes.
Well, this is its confidence and supply agreement with the Tories multiplied 100 times, with icing and candles on it, and then some more.
Sinn Fein won't have influence with the Dublin government, it is poised to be the government.
Mary Lou McDonald not just as Tanaiste, but possibly as Taoiseach, if an agreement is reached with Fianna Fail to rotate the position. Unionists have lost their majority in the Assembly and at Westminster. We are heading towards a border in the Irish Sea. Sinn Fein taking power in Dublin is the last thing Arlene Foster needed.
When the DUP leader talked recently about rebuilding relations with the Republic, she will have had envisaged doing so with Micheal Martin. Never will she have considered it could be Micheal and Mary Lou.
The constitutional status of Northern Ireland won't change until a majority here agrees. Sinn Fein can't alter that but it will no longer be shouting about Irish unity from the sidelines.
It will be able to put the issue right, left and centre of the political stage.
It will push London for a border poll in the next five years - although Boris Johnson is as likely to agree to that as he is to build a bridge to Scotland.
Sinn Fein aims to establish a parliamentary committee and citizens' assembly to plan for Irish unity.
There will be unprecedented pressures on the party from its base that this isn't just a talking shop.
Expectations will likely grow of more progress towards a united Ireland than is realistic.
But unionists will still be unnerved.
Until now, Ms McDonald has been one of five local party leaders speaking to Mr Johnson. She's on course now to meet him as a co-equal.
The psychological impact of that on unionists can't be under-estimated.
She will have influence in the Brexit trade talks with Britain.
Of course, success carries dangers for Sinn Fein. Moving from being a party of protest to one of power is never easy. It will have to carefully negotiate a programme for government with Fianna Fail. The Shinners' reputation for being skilled negotiators doesn't actually stack up when the evidence is examined on this side of the border.
The money needed for all the grandiose pledges in New Decade, New Approach wasn't even secured from London. Before the party brought down the Stormont institutions in 2017, the DUP had run rings around it in government for a decade. If it was a battle a day, the Shinners lost 365 every year which led to disillusioned supporters and lost votes.
The party didn't deliver same-sex marriage or abortion reform - Westminster MPs did. Neither did it effect economic change either.
It has been able to blame the Brits for that, arguing that it was working within the confines of Tory austerity. That won't wash when it's in power in Dublin. The buck will stop with Mary Lou. Sinn Fein has an impressive team down South. The talent that saw it win the election must now focus on make a success of governing.