The Windhovers' Call

Xellos says to the Tiefling in infernal

 He points a finger at the Tiefling and 3 bolts fly while still maintaining his concentration on his dual Haste on Kurt & Frandal

Magic Missile damage: 13 force

Everyone: A summary of damage taken by party members and monsters thus far:

• Kurt: 28 hit points lost (10 piercing; 18 force)
• Arissa: 5 hit points lost (5 force)
• Wight: 7 hit points (7 cold)
• Tiefling: 39 hit points (8 piercing; 18 slashing; 13 force)
• Ozum, Frandal, and Xellos: no damage taken

Spoiler for Xellos: 

 Ozum: Your action is next (and you will continue to follow Xellos in the initiative order). Make sure you identify where in the room you will be standing during your turn. 

Xellos immediately says (in common), "The Tiefling is leaving the wight for Kurt!"

Ozum moves past Xellos and Arissa (28) and pulls out a sprig of mistletoe. He begins a drudic chant and rhythmically moves his arms as he sways side to side. As he does, time seems to distort around him first fast then very slow. The effect migrates across the room stopping on the (enemy) tiefling and wight. He then looks at the owl on his shoulder and says, "Get 'em, Tootsie" as he points at the hostile tiefling.

Ozum casts slow on the tiefling and the wight then orders the bag of tricks owl to attack the tiefling (next turn).

WIS saving throws for wight (8 and tiefling 11

The wight has failed its saving throw and is now under the effect of Ozum's Slow spell (for 1 minute or until Ozum's concentration is broken). The tiefling, however, shrugs off the failed saving throw and wills himself to override the effect of the spell. He then takes a reaction turn and casts Ray of Frost at Ozum. Ranged attack: 23 Damage: 25

The tiefling's spell sorely misses Ozum, and the blue-white light that was intended for the gnome flashes over the heads of Ozum, Arissa, and Xellos and explodes spectacularly, but harmlessly, into the stone wall. 

Frandal: Your turn is next.

Realizing that his arrows had no effect due to a protective spell he could overcome by hitting the tiefling, but due to his undead nature, Frandal changes his strategy. Now that Kurt is engaged in melee with the wright it was too dangerous to use the same spell. He, however, had some tricks left. He cames out over the other side of the column, and takes aim against the Tiefling. 

This time, the arrow burts in flames as it leaves the bow looking for the Tiefling’s chest. 

Then he fires another one, thanks to his enhanced speed, to the other undead. 

Kurt continues to engage the tiefling.

Frandal's attacks on the tiefling fail to do any damage, but he succeeds in piercing the wight squarely in the chest with an arrow. The wight staggers backward a step or two but clings with both hands to its longsword, still committed to the task of facing Kurt...

(20 points total damage to the wight now)

Kurt: The next turn is yours.

Kurt, in a flurry of brilliant swordsmanship, lands five consecutive slashing blows on the tiefling. Each slash of the scimitar causes the tiefling to take a step backward as he ineffectively attempts to shield himself from the steel blade. Enraged at the opposition he is facing, the tiefling takes another reaction and reaches out and places his hand on Kurt's arm; in an attempt to rid himself of the scimitar-wielding nuisance, the tiefling uses his paralyzing touch ability. (Attack: 29 Damage: 13)

The tiefling is mystified that he has somehow utterly failed to paralyze his foe. (In total, the tiefling has lost 88 hit points thus far).

Arissa: Your action is next. 

"How could you possibly have been so stupid as to think that Bharash himself would deign to confront someone as insignificant as you?" Arissa casts vicious mockery (Damage: 3) on the tiefling (WIS 14), then says a healing word (2nd level) to Kurt, restoring 6 HP.

WIS save for tiefling: 18

The tiefling begins to speak softly while he steps toward the wight. He then raises his staff slightly while making a small spiral motion with it. Suddenly, where the tiefling and wight stood you see nothing more than a small flash of light and a thin wisp of smoke. Via Dimension Door, the two undead creatures have seemingly left the five of you alone in the keep.

If you elect to move to other parts of the keep, use this map: 

 “ I hate when they do that..” says Frandal

Frandal: 

Quote from Jack_Joke >>

WIS save for tiefling: 18

The tiefling begins to speak softly while he steps toward the wight. He then raises his staff slightly while making a small spiral motion with it. Suddenly, where the tiefling and wight stood you see a small flash of light. Via Dimension Door, the two undead creatures attempt to leave the five of you alone in the keep. Arissa Antal intervenes in the tiefling's effort to escape by casting Counterspell; therefore, rather than finding himself safe from danger, the tiefling remains in the keep, forced to face further assault from all of you. Being able to do nothing more than move, the tiefling scurries to the other side of the pillar to ensure that he is, as much as possible, out of the line of fire from all of you. 

EDIT: The above reflects the chain of events brought about by Arissa's casting of Counterspell to prevent the tiefling and wight from teleporting out of the keep. So, the map below is the best depiction of where everyone is located in the room.

Xellos: Your turn is next, followed by Ozum's.

Xellos says, "Time to finish this. Now you die!"

As their spell was interrupted, he started chanting, as he gives this long winded incantation, a massive ball of fire begins coalescing in his hands over his head, "Za'zam za'zamore T'almat Golam! Burning in the depths in the pits of darkness let the fires of Hell take you! Let the fire become my sword and strike you down—Venom!" 

The massive ball of fire is hurled to box 19 to blast both the Tiefling and the wight while spare Kurt from the damage.

Fireball damage: 31 (DEX ST 16 for half)

DEX saving throw for tiefling: 8 | DEX saving throw for wight: 17 (Both sustain the full 31 points of fire damage from Xellos' spell)

Everyone: The tiefling, despite being engulfed in flames, can be heard chuckling and saying something in Infernal. When the fire dissipates, the tiefling's corpse can be seen lying in a crumpled heap, its robes nearly completely consumed by the flames. The tiefling's thin, pale staff, badly singed lies next to him. The wight, still alive, looks at the tiefling's lifeless corpse, clutches its longsword in both hands and begins to walk slowly toward all of you, seemingly ready to lose its "life" in one last clash. 

Ozum: Your turn is next, followed by Frandal's. 

Spoiler for Xellos: 

 NOTE: The original die rolls above were both fails. After editing, however, they changed.

Ozum shouts "Shillelagh!" as he slams his staff and waves his mistletoe above it. The smooth staff grows large gnarled wooden knots and a big club head as the gnome rushes to the wight. He swings once at the undead's knee.

Attack: 16 Damage: 12

Ozum's quarterstaff blow buckles the wight's leg and causes the creature to slam face-down into the stone floor. The burning light in its eyes slowly fades away. The undead is now dead. 

Much of the keep still remains before you:

"Shall we explore?"

Arissa opens the door leading to E, and, assuming she sees nothing obviously dangerous, enters and looks around.

“Well done everyone but I am concerned about what we’ve learned. That creature “ he points to the tiefling “he remind me some kind of lich, I think that we will face it again, perhaps in another body. I was waiting for Bharash to come here, and that it was us probe that this plan was wise. But I am afraid that we have show to our enemies now, to this creature and to whoever he was refering. Let’s go try to find any clue of Ro Vena and her whereabouts. For I still have hope that she had escaped this creature and we could find her. “ he turns and look to the others “ Should we split to cover more in less time?”